# This file is a part of Julia. License is MIT: https://julialang.org/license

function show(io::IO, ::MIME"text/plain", u::UndefInitializer)
    show(io, u)
    get(io, :compact, false) && return
    print(io, ": array initializer with undefined values")
end

# first a few multiline show functions for types defined before the MIME type:

show(io::IO, ::MIME"text/plain", r::AbstractRange) = show(io, r) # always use the compact form for printing ranges

function show(io::IO, ::MIME"text/plain", r::LinRange)
    isempty(r) && return show(io, r)
    # show for LinRange, e.g.
    # range(1, stop=3, length=7)
    # 7-element LinRange{Float64}:
    #   1.0,1.33333,1.66667,2.0,2.33333,2.66667,3.0
    summary(io, r)
    println(io, ":")
    print_range(io, r)
end

function show(io::IO, ::MIME"text/plain", f::Function)
    get(io, :compact, false) && return show(io, f)
    ft = typeof(f)
    mt = ft.name.mt
    if isa(f, Core.IntrinsicFunction)
        print(io, f)
        id = Core.Intrinsics.bitcast(Int32, f)
        print(io, " (intrinsic function #$id)")
    elseif isa(f, Core.Builtin)
        print(io, mt.name, " (built-in function)")
    else
        name = mt.name
        isself = isdefined(ft.name.module, name) &&
                 ft == typeof(getfield(ft.name.module, name))
        n = length(methods(f))
        m = n==1 ? "method" : "methods"
        sname = string(name)
        ns = (isself || '#' in sname) ? sname : string("(::", ft, ")")
        what = startswith(ns, '@') ? "macro" : "generic function"
        print(io, ns, " (", what, " with $n $m)")
    end
end

show(io::IO, ::MIME"text/plain", c::ComposedFunction) = show(io, c)
show(io::IO, ::MIME"text/plain", c::Returns) = show(io, c)

function show(io::IO, ::MIME"text/plain", iter::Union{KeySet,ValueIterator})
    isempty(iter) && get(io, :compact, false) && return show(io, iter)
    summary(io, iter)
    isempty(iter) && return
    print(io, ". ", isa(iter,KeySet) ? "Keys" : "Values", ":")
    limit = get(io, :limit, false)::Bool
    if limit
        sz = displaysize(io)
        rows, cols = sz[1] - 3, sz[2]
        rows < 2 && (print(io, " …"); return)
        cols < 4 && (cols = 4)
        cols -= 2 # For prefix "  "
        rows -= 1 # For summary
    else
        rows = cols = typemax(Int)
    end

    for (i, v) in enumerate(iter)
        print(io, "\n  ")
        i == rows < length(iter) && (print(io, "⋮"); break)

        if limit
            str = sprint(show, v, context=io, sizehint=0)
            str = _truncate_at_width_or_chars(str, cols, "\r\n")
            print(io, str)
        else
            show(io, v)
        end
    end
end

function show(io::IO, ::MIME"text/plain", t::AbstractDict{K,V}) where {K,V}
    isempty(t) && return show(io, t)
    # show more descriptively, with one line per key/value pair
    recur_io = IOContext(io, :SHOWN_SET => t)
    limit = get(io, :limit, false)::Bool
    if !haskey(io, :compact)
        recur_io = IOContext(recur_io, :compact => true)
    end
    recur_io_k = IOContext(recur_io, :typeinfo=>keytype(t))
    recur_io_v = IOContext(recur_io, :typeinfo=>valtype(t))

    summary(io, t)
    isempty(t) && return
    print(io, ":")
    show_circular(io, t) && return
    if limit
        sz = displaysize(io)
        rows, cols = sz[1] - 3, sz[2]
        rows < 2   && (print(io, " …"); return)
        cols < 12  && (cols = 12) # Minimum widths of 2 for key, 4 for value
        cols -= 6 # Subtract the widths of prefix "  " separator " => "
        rows -= 1 # Subtract the summary

        # determine max key width to align the output, caching the strings
        ks = Vector{String}(undef, min(rows, length(t)))
        vs = Vector{String}(undef, min(rows, length(t)))
        keylen = 0
        vallen = 0
        for (i, (k, v)) in enumerate(t)
            i > rows && break
            ks[i] = sprint(show, k, context=recur_io_k, sizehint=0)
            vs[i] = sprint(show, v, context=recur_io_v, sizehint=0)
            keylen = clamp(length(ks[i]), keylen, cols)
            vallen = clamp(length(vs[i]), vallen, cols)
        end
        if keylen > max(div(cols, 2), cols - vallen)
            keylen = max(cld(cols, 3), cols - vallen)
        end
    else
        rows = cols = typemax(Int)
    end

    for (i, (k, v)) in enumerate(t)
        print(io, "\n  ")
        if i == rows < length(t)
            print(io, rpad("⋮", keylen), " => ⋮")
            break
        end

        if limit
            key = rpad(_truncate_at_width_or_chars(ks[i], keylen, "\r\n"), keylen)
        else
            key = sprint(show, k, context=recur_io_k, sizehint=0)
        end
        print(recur_io, key)
        print(io, " => ")

        if limit
            val = _truncate_at_width_or_chars(vs[i], cols - keylen, "\r\n")
            print(io, val)
        else
            show(recur_io_v, v)
        end
    end
end

function summary(io::IO, t::AbstractSet)
    n = length(t)
    showarg(io, t, true)
    print(io, " with ", n, (n==1 ? " element" : " elements"))
end

function show(io::IO, ::MIME"text/plain", t::AbstractSet{T}) where T
    isempty(t) && return show(io, t)
    # show more descriptively, with one line per value
    recur_io = IOContext(io, :SHOWN_SET => t)
    limit = get(io, :limit, false)::Bool

    summary(io, t)
    isempty(t) && return
    print(io, ":")
    show_circular(io, t) && return
    if limit
        sz = displaysize(io)
        rows, cols = sz[1] - 3, sz[2]
        rows < 2   && (print(io, " …"); return)
        cols -= 2 # Subtract the width of prefix "  "
        cols < 4  && (cols = 4) # Minimum widths of 4 for value
        rows -= 1 # Subtract the summary
    else
        rows = cols = typemax(Int)
    end

    for (i, v) in enumerate(t)
        print(io, "\n  ")
        if i == rows < length(t)
            print(io, rpad("⋮", 2))
            break
        end

        if limit
            str = sprint(show, v, context=recur_io, sizehint=0)
            print(io, _truncate_at_width_or_chars(str, cols, "\r\n"))
        else
            show(recur_io, v)
        end
    end
end

function show(io::IO, ::MIME"text/plain", opt::JLOptions)
    println(io, "JLOptions(")
    fields = fieldnames(JLOptions)
    nfields = length(fields)
    for (i, f) in enumerate(fields)
        v = getfield(opt, i)
        if isa(v, Ptr{UInt8})
            v = (v != C_NULL) ? unsafe_string(v) : ""
        elseif isa(v, Ptr{Ptr{UInt8}})
            v = unsafe_load_commands(v)
        end
        println(io, "  ", f, " = ", repr(v), i < nfields ? "," : "")
    end
    print(io, ")")
end

function show(io::IO, ::MIME"text/plain", t::Task)
    show(io, t)
    if istaskfailed(t)
        println(io)
        show_task_exception(io, t, indent = false)
    end
end


print(io::IO, s::Symbol) = (write(io,s); nothing)

"""
    IOContext

`IOContext` provides a mechanism for passing output configuration settings among [`show`](@ref) methods.

In short, it is an immutable dictionary that is a subclass of `IO`. It supports standard
dictionary operations such as [`getindex`](@ref), and can also be used as an I/O stream.
"""
struct IOContext{IO_t <: IO} <: AbstractPipe
    io::IO_t
    dict::ImmutableDict{Symbol, Any}

    function IOContext{IO_t}(io::IO_t, dict::ImmutableDict{Symbol, Any}) where IO_t<:IO
        @assert !(IO_t <: IOContext) "Cannot create `IOContext` from another `IOContext`."
        return new(io, dict)
    end
end

# (Note that TTY and TTYTerminal io types have a :color property.)
unwrapcontext(io::IO) = io, get(io,:color,false) ? ImmutableDict{Symbol,Any}(:color, true) : ImmutableDict{Symbol,Any}()
unwrapcontext(io::IOContext) = io.io, io.dict

function IOContext(io::IO, dict::ImmutableDict)
    io0 = unwrapcontext(io)[1]
    IOContext{typeof(io0)}(io0, dict)
end

convert(::Type{IOContext}, io::IO) = IOContext(unwrapcontext(io)...)

IOContext(io::IO) = convert(IOContext, io)

function IOContext(io::IO, KV::Pair)
    io0, d = unwrapcontext(io)
    IOContext(io0, ImmutableDict{Symbol,Any}(d, KV[1], KV[2]))
end

"""
    IOContext(io::IO, context::IOContext)

Create an `IOContext` that wraps an alternate `IO` but inherits the properties of `context`.
"""
IOContext(io::IO, context::IO) = IOContext(unwrapcontext(io)[1], unwrapcontext(context)[2])

"""
    IOContext(io::IO, KV::Pair...)

Create an `IOContext` that wraps a given stream, adding the specified `key=>value` pairs to
the properties of that stream (note that `io` can itself be an `IOContext`).

 - use `(key => value) in io` to see if this particular combination is in the properties set
 - use `get(io, key, default)` to retrieve the most recent value for a particular key

The following properties are in common use:

 - `:compact`: Boolean specifying that values should be printed more compactly, e.g.
   that numbers should be printed with fewer digits. This is set when printing array
   elements. `:compact` output should not contain line breaks.
 - `:limit`: Boolean specifying that containers should be truncated, e.g. showing `…` in
   place of most elements.
 - `:displaysize`: A `Tuple{Int,Int}` giving the size in rows and columns to use for text
   output. This can be used to override the display size for called functions, but to
   get the size of the screen use the `displaysize` function.
 - `:typeinfo`: a `Type` characterizing the information already printed
   concerning the type of the object about to be displayed. This is mainly useful when
   displaying a collection of objects of the same type, so that redundant type information
   can be avoided (e.g. `[Float16(0)]` can be shown as "Float16[0.0]" instead
   of "Float16[Float16(0.0)]" : while displaying the elements of the array, the `:typeinfo`
   property will be set to `Float16`).
 - `:color`: Boolean specifying whether ANSI color/escape codes are supported/expected.
   By default, this is determined by whether `io` is a compatible terminal and by any
   `--color` command-line flag when `julia` was launched.

# Examples

```jldoctest
julia> io = IOBuffer();

julia> printstyled(IOContext(io, :color => true), "string", color=:red)

julia> String(take!(io))
"\\e[31mstring\\e[39m"

julia> printstyled(io, "string", color=:red)

julia> String(take!(io))
"string"
```

```jldoctest
julia> print(IOContext(stdout, :compact => false), 1.12341234)
1.12341234
julia> print(IOContext(stdout, :compact => true), 1.12341234)
1.12341
```

```jldoctest
julia> function f(io::IO)
           if get(io, :short, false)
               print(io, "short")
           else
               print(io, "loooooong")
           end
       end
f (generic function with 1 method)

julia> f(stdout)
loooooong
julia> f(IOContext(stdout, :short => true))
short
```
"""
IOContext(io::IO, KV::Pair, KVs::Pair...) = IOContext(IOContext(io, KV), KVs...)

show(io::IO, ctx::IOContext) = (print(io, "IOContext("); show(io, ctx.io); print(io, ")"))

pipe_reader(io::IOContext) = io.io
pipe_writer(io::IOContext) = io.io
lock(io::IOContext) = lock(io.io)
unlock(io::IOContext) = unlock(io.io)

in(key_value::Pair, io::IOContext) = in(key_value, io.dict, ===)
in(key_value::Pair, io::IO) = false
haskey(io::IOContext, key) = haskey(io.dict, key)
haskey(io::IO, key) = false
getindex(io::IOContext, key) = getindex(io.dict, key)
getindex(io::IO, key) = throw(KeyError(key))
get(io::IOContext, key, default) = get(io.dict, key, default)
get(io::IO, key, default) = default
keys(io::IOContext) = keys(io.dict)
keys(io::IO) = keys(ImmutableDict{Symbol,Any}())

displaysize(io::IOContext) = haskey(io, :displaysize) ? io[:displaysize]::Tuple{Int,Int} : displaysize(io.io)

show_circular(io::IO, @nospecialize(x)) = false
function show_circular(io::IOContext, @nospecialize(x))
    d = 1
    for (k, v) in io.dict
        if k === :SHOWN_SET
            if v === x
                print(io, "#= circular reference @-$d =#")
                return true
            end
            d += 1
        end
    end
    return false
end

"""
    show([io::IO = stdout], x)

Write a text representation of a value `x` to the output stream `io`. New types `T`
should overload `show(io::IO, x::T)`. The representation used by `show` generally
includes Julia-specific formatting and type information, and should be parseable
Julia code when possible.

[`repr`](@ref) returns the output of `show` as a string.

To customize human-readable text output for objects of type `T`, define
`show(io::IO, ::MIME"text/plain", ::T)` instead. Checking the `:compact`
[`IOContext`](@ref) property of `io` in such methods is recommended,
since some containers show their elements by calling this method with
`:compact => true`.

See also [`print`](@ref), which writes un-decorated representations.

# Examples
```jldoctest
julia> show("Hello World!")
"Hello World!"
julia> print("Hello World!")
Hello World!
```
"""
show(io::IO, @nospecialize(x)) = show_default(io, x)

show(x) = show(stdout, x)

# avoid inferring show_default on the type of `x`
show_default(io::IO, @nospecialize(x)) = _show_default(io, inferencebarrier(x))

function _show_default(io::IO, @nospecialize(x))
    t = typeof(x)
    show(io, inferencebarrier(t)::DataType)
    print(io, '(')
    nf = nfields(x)
    nb = sizeof(x)::Int
    if nf != 0 || nb == 0
        if !show_circular(io, x)
            recur_io = IOContext(io, Pair{Symbol,Any}(:SHOWN_SET, x),
                                 Pair{Symbol,Any}(:typeinfo, Any))
            for i in 1:nf
                f = fieldname(t, i)
                if !isdefined(x, f)
                    print(io, undef_ref_str)
                else
                    show(recur_io, getfield(x, i))
                end
                if i < nf
                    print(io, ", ")
                end
            end
        end
    else
        print(io, "0x")
        r = Ref{Any}(x)
        GC.@preserve r begin
            p = unsafe_convert(Ptr{Cvoid}, r)
            for i in (nb - 1):-1:0
                print(io, string(unsafe_load(convert(Ptr{UInt8}, p + i)), base = 16, pad = 2))
            end
        end
    end
    print(io,')')
end

# Check if a particular symbol is exported from a standard library module
function is_exported_from_stdlib(name::Symbol, mod::Module)
    !isdefined(mod, name) && return false
    orig = getfield(mod, name)
    while !(mod === Base || mod === Core)
        parent = parentmodule(mod)
        if mod === Main || mod === parent || parent === Main
            return false
        end
        mod = parent
    end
    return isexported(mod, name) && isdefined(mod, name) && !isdeprecated(mod, name) && getfield(mod, name) === orig
end

function show_function(io::IO, f::Function, compact::Bool)
    ft = typeof(f)
    mt = ft.name.mt
    if mt === Symbol.name.mt
        # uses shared method table
        show_default(io, f)
    elseif compact
        print(io, mt.name)
    elseif isdefined(mt, :module) && isdefined(mt.module, mt.name) &&
        getfield(mt.module, mt.name) === f
        if is_exported_from_stdlib(mt.name, mt.module) || mt.module === Main
            show_sym(io, mt.name)
        else
            print(io, mt.module, ".")
            show_sym(io, mt.name)
        end
    else
        show_default(io, f)
    end
end

show(io::IO, f::Function) = show_function(io, f, get(io, :compact, false)::Bool)
print(io::IO, f::Function) = show_function(io, f, true)

function show(io::IO, f::Core.IntrinsicFunction)
    if !(get(io, :compact, false)::Bool)
        print(io, "Core.Intrinsics.")
    end
    print(io, nameof(f))
end

print(io::IO, f::Core.IntrinsicFunction) = print(io, nameof(f))

show(io::IO, ::Core.TypeofBottom) = print(io, "Union{}")
show(io::IO, ::MIME"text/plain", ::Core.TypeofBottom) = print(io, "Union{}")

function print_without_params(@nospecialize(x))
    b = unwrap_unionall(x)
    return isa(b, DataType) && b.name.wrapper === x
end

has_typevar(@nospecialize(t), v::TypeVar) = ccall(:jl_has_typevar, Cint, (Any, Any), t, v)!=0

function io_has_tvar_name(io::IOContext, name::Symbol, @nospecialize(x))
    for (key, val) in io.dict
        if key === :unionall_env && val isa TypeVar && val.name === name && has_typevar(x, val)
            return true
        end
    end
    return false
end
io_has_tvar_name(io::IO, name::Symbol, @nospecialize(x)) = false

modulesof!(s::Set{Module}, x::TypeVar) = modulesof!(s, x.ub)
function modulesof!(s::Set{Module}, x::Type)
    x = unwrap_unionall(x)
    if x isa DataType
        push!(s, x.name.module)
    elseif x isa Union
        modulesof!(s, x.a)
        modulesof!(s, x.b)
    end
    s
end

# given an IO context for printing a type, reconstruct the proper type that
# we're attempting to represent.
# Union{T} where T is a degenerate case and is equal to T.ub, but we don't want
# to print them that way, so filter those out from our aliases completely.
function makeproper(io::IO, @nospecialize(x::Type))
    if io isa IOContext
        for (key, val) in io.dict
            if key === :unionall_env && val isa TypeVar
                x = UnionAll(val, x)
            end
        end
    end
    has_free_typevars(x) && return Any
    return x
end

function make_typealias(@nospecialize(x::Type))
    Any === x && return nothing
    x <: Tuple && return nothing
    mods = modulesof!(Set{Module}(), x)
    Core in mods && push!(mods, Base)
    aliases = Tuple{GlobalRef,SimpleVector}[]
    xenv = UnionAll[]
    for p in uniontypes(unwrap_unionall(x))
        p isa UnionAll && push!(xenv, p)
    end
    x isa UnionAll && push!(xenv, x)
    for mod in mods
        for name in names(mod)
            if isdefined(mod, name) && !isdeprecated(mod, name) && isconst(mod, name)
                alias = getfield(mod, name)
                if alias isa Type && !has_free_typevars(alias) && !print_without_params(alias) && x <: alias
                    if alias isa UnionAll
                        (ti, env) = ccall(:jl_type_intersection_with_env, Any, (Any, Any), x, alias)::SimpleVector
                        # ti === Union{} && continue # impossible, since we already checked that x <: alias
                        env = env::SimpleVector
                        # TODO: In some cases (such as the following), the `env` is over-approximated.
                        #       We'd like to disable `fix_inferred_var_bound` since we'll already do that fix-up here.
                        #       (or detect and reverse the compution of it here).
                        #   T = Array{Array{T,1}, 1} where T
                        #   (ti, env) = ccall(:jl_type_intersection_with_env, Any, (Any, Any), T, Vector)
                        #   env[1].ub.var == T.var
                        applied = try
                                # this can fail if `x` contains a covariant
                                # union, and the non-matching branch of the
                                # union has additional restrictions on the
                                # bounds of the environment that are not met by
                                # the instantiation found above
                                alias{env...}
                            catch ex
                                ex isa TypeError || rethrow()
                                continue
                            end
                        for p in xenv
                            applied = rewrap_unionall(applied, p)
                        end
                        has_free_typevars(applied) && continue
                        applied === x || continue # it couldn't figure out the parameter matching
                    elseif alias === x
                        env = Core.svec()
                    else
                        continue # not a complete match
                    end
                    push!(aliases, (GlobalRef(mod, name), env))
                end
            end
        end
    end
    if length(aliases) == 1 # TODO: select the type with the "best" (shortest?) environment
        return aliases[1]
    end
end

isgensym(s::Symbol) = '#' in string(s)

function show_can_elide(p::TypeVar, wheres::Vector, elide::Int, env::SimpleVector, skip::Int)
    elide == 0 && return false
    wheres[elide] === p || return false
    for i = (elide + 1):length(wheres)
        v = wheres[i]::TypeVar
        has_typevar(v.lb, p) && return false
        has_typevar(v.ub, p) && return false
    end
    for i = 1:length(env)
        i == skip && continue
        has_typevar(env[i], p) && return false
    end
    return true
end

function show_typeparams(io::IO, env::SimpleVector, orig::SimpleVector, wheres::Vector)
    n = length(env)
    elide = length(wheres)
    function egal_var(p::TypeVar, @nospecialize o)
        return o isa TypeVar &&
            ccall(:jl_types_egal, Cint, (Any, Any), p.ub, o.ub) != 0 &&
            ccall(:jl_types_egal, Cint, (Any, Any), p.lb, o.lb) != 0
    end
    for i = n:-1:1
        p = env[i]
        if p isa TypeVar
            if i == n && egal_var(p, orig[i]) && show_can_elide(p, wheres, elide, env, i)
                n -= 1
                elide -= 1
            elseif p.lb === Union{} && isgensym(p.name) && show_can_elide(p, wheres, elide, env, i)
                elide -= 1
            elseif p.ub === Any && isgensym(p.name) && show_can_elide(p, wheres, elide, env, i)
                elide -= 1
            end
        end
    end
    if n > 0
        print(io, "{")
        for i = 1:n
            p = env[i]
            if p isa TypeVar
                if p.lb === Union{} && something(findfirst(@nospecialize(w) -> w === p, wheres), 0) > elide
                    print(io, "<:")
                    show(io, p.ub)
                elseif p.ub === Any && something(findfirst(@nospecialize(w) -> w === p, wheres), 0) > elide
                    print(io, ">:")
                    show(io, p.lb)
                else
                    show(io, p)
                end
            else
                show(io, p)
            end
            i < n && print(io, ", ")
        end
        print(io, "}")
    end
    resize!(wheres, elide)
    nothing
end

function show_typealias(io::IO, name::GlobalRef, x::Type, env::SimpleVector, wheres::Vector)
    if !(get(io, :compact, false)::Bool)
        # Print module prefix unless alias is visible from module passed to
        # IOContext. If :module is not set, default to Main. nothing can be used
        # to force printing prefix.
        from = get(io, :module, Main)
        if (from === nothing || !isvisible(name.name, name.mod, from))
            show(io, name.mod)
            print(io, ".")
        end
    end
    print(io, name.name)
    isempty(env) && return
    io = IOContext(io)
    for p in wheres
        io = IOContext(io, :unionall_env => p)
    end
    orig = getfield(name.mod, name.name)
    vars = TypeVar[]
    while orig isa UnionAll
        push!(vars, orig.var)
        orig = orig.body
    end
    show_typeparams(io, env, Core.svec(vars...), wheres)
    nothing
end

function make_wheres(io::IO, env::SimpleVector, @nospecialize(x::Type))
    seen = IdSet()
    wheres = TypeVar[]
    # record things printed by the context
    if io isa IOContext
        for (key, val) in io.dict
            if key === :unionall_env && val isa TypeVar && has_typevar(x, val)
                push!(seen, val)
            end
        end
    end
    # record things in x to print outermost
    while x isa UnionAll
        if !(x.var in seen)
            push!(seen, x.var)
            push!(wheres, x.var)
        end
        x = x.body
    end
    # record remaining things in env to print innermost
    for i = length(env):-1:1
        p = env[i]
        if p isa TypeVar && !(p in seen)
            push!(seen, p)
            pushfirst!(wheres, p)
        end
    end
    return wheres
end

function show_wheres(io::IO, wheres::Vector{TypeVar})
    isempty(wheres) && return
    io = IOContext(io)
    n = length(wheres)
    for i = 1:n
        p = wheres[i]
        print(io, n == 1 ? " where " : i == 1 ? " where {" : ", ")
        show(io, p)
        io = IOContext(io, :unionall_env => p)
    end
    n > 1 && print(io, "}")
    nothing
end

function show_typealias(io::IO, @nospecialize(x::Type))
    properx = makeproper(io, x)
    alias = make_typealias(properx)
    alias === nothing && return false
    wheres = make_wheres(io, alias[2], x)
    show_typealias(io, alias[1], x, alias[2], wheres)
    show_wheres(io, wheres)
    return true
end

function make_typealiases(@nospecialize(x::Type))
    aliases = SimpleVector[]
    Any === x && return aliases, Union{}
    x <: Tuple && return aliases, Union{}
    mods = modulesof!(Set{Module}(), x)
    Core in mods && push!(mods, Base)
    vars = Dict{Symbol,TypeVar}()
    xenv = UnionAll[]
    each = Any[]
    for p in uniontypes(unwrap_unionall(x))
        p isa UnionAll && push!(xenv, p)
        push!(each, rewrap_unionall(p, x))
    end
    x isa UnionAll && push!(xenv, x)
    for mod in mods
        for name in names(mod)
            if isdefined(mod, name) && !isdeprecated(mod, name) && isconst(mod, name)
                alias = getfield(mod, name)
                if alias isa Type && !has_free_typevars(alias) && !print_without_params(alias) && !(alias <: Tuple)
                    (ti, env) = ccall(:jl_type_intersection_with_env, Any, (Any, Any), x, alias)::SimpleVector
                    ti === Union{} && continue
                    # make sure this alias wasn't from an unrelated part of the Union
                    mod2 = modulesof!(Set{Module}(), alias)
                    mod in mod2 || (mod === Base && Core in mods) || continue
                    env = env::SimpleVector
                    applied = alias
                    if !isempty(env)
                        applied = try
                                # this can fail if `x` contains a covariant
                                # union, and the non-matching branch of the
                                # union has additional restrictions on the
                                # bounds of the environment that are not met by
                                # the instantiation found above
                                alias{env...}
                            catch ex
                                ex isa TypeError || rethrow()
                                continue
                            end
                    end
                    ul = unionlen(applied)
                    for p in xenv
                        applied = rewrap_unionall(applied, p)
                    end
                    has_free_typevars(applied) && continue
                    applied <: x || continue # parameter matching didn't make a subtype
                    print_without_params(x) && (env = Core.svec())
                    for typ in each # check that the alias also fully subsumes at least component of the input
                        if typ <: applied
                            push!(aliases, Core.svec(GlobalRef(mod, name), env, applied, (ul, -length(env))))
                            break
                        end
                    end
                end
            end
        end
    end
    if isempty(aliases)
        return aliases, Union{}
    end
    sort!(aliases, by = x -> x[4]::Tuple{Int,Int}, rev = true) # heuristic sort by "best" environment
    let applied = Union{}
        applied1 = Union{}
        keep = SimpleVector[]
        prev = (0, 0)
        for alias in aliases
            alias4 = alias[4]::Tuple{Int,Int}
            if alias4[1] < 2
                if !(alias[3] <: applied)
                    applied1 = Union{applied1, alias[3]}
                    push!(keep, alias)
                end
            elseif alias4 == prev || !(alias[3] <: applied)
                applied = applied1 = Union{applied1, alias[3]}
                push!(keep, alias)
                prev = alias4
            end
        end
        return keep, applied1
    end
end

function show_unionaliases(io::IO, x::Union)
    properx = makeproper(io, x)
    aliases, applied = make_typealiases(properx)
    isempty(aliases) && return false
    first = true
    tvar = false
    for typ in uniontypes(x)
        if isa(typ, TypeVar)
            tvar = true # sort bare TypeVars to the end
            continue
        elseif rewrap_unionall(typ, properx) <: applied
            continue
        end
        print(io, first ? "Union{" : ", ")
        first = false
        show(io, typ)
    end
    if first && !tvar && length(aliases) == 1
        alias = aliases[1]
        env = alias[2]::SimpleVector
        wheres = make_wheres(io, env, x)
        show_typealias(io, alias[1], x, env, wheres)
        show_wheres(io, wheres)
    else
        for alias in aliases
            print(io, first ? "Union{" : ", ")
            first = false
            env = alias[2]::SimpleVector
            wheres = make_wheres(io, env, x)
            show_typealias(io, alias[1], x, env, wheres)
            show_wheres(io, wheres)
        end
        if tvar
            for typ in uniontypes(x)
                if isa(typ, TypeVar)
                    print(io, ", ")
                    show(io, typ)
                end
            end
        end
        print(io, "}")
    end
    return true
end

function show(io::IO, ::MIME"text/plain", @nospecialize(x::Type))
    if !print_without_params(x)
        properx = makeproper(io, x)
        if make_typealias(properx) !== nothing || (unwrap_unionall(x) isa Union && x <: make_typealiases(properx)[2])
            show(IOContext(io, :compact => true), x)
            if !(get(io, :compact, false)::Bool)
                printstyled(io, " (alias for "; color = :light_black)
                printstyled(IOContext(io, :compact => false), x, color = :light_black)
                printstyled(io, ")"; color = :light_black)
            end
            return
        end
    end
    show(io, x)
    # give a helpful hint for function types
    if x isa DataType && x !== UnionAll && !(get(io, :compact, false)::Bool)
        tn = x.name::Core.TypeName
        globname = isdefined(tn, :mt) ? tn.mt.name : nothing
        if is_global_function(tn, globname)
            print(io, " (singleton type of function ")
            show_sym(io, globname)
            print(io, ", subtype of Function)")
        end
    end
end

show(io::IO, @nospecialize(x::Type)) = _show_type(io, inferencebarrier(x))
function _show_type(io::IO, @nospecialize(x::Type))
    if print_without_params(x)
        show_type_name(io, (unwrap_unionall(x)::DataType).name)
        return
    elseif get(io, :compact, true) && show_typealias(io, x)
        return
    elseif x isa DataType
        show_datatype(io, x)
        return
    elseif x isa Union
        if get(io, :compact, true) && show_unionaliases(io, x)
            return
        end
        print(io, "Union")
        show_delim_array(io, uniontypes(x), '{', ',', '}', false)
        return
    end

    x = x::UnionAll
    wheres = TypeVar[]
    let io = IOContext(io)
        while x isa UnionAll
            var = x.var
            if var.name === :_ || io_has_tvar_name(io, var.name, x)
                counter = 1
                while true
                    newname = Symbol(var.name, counter)
                    if !io_has_tvar_name(io, newname, x)
                        var = TypeVar(newname, var.lb, var.ub)
                        x = x{var}
                        break
                    end
                    counter += 1
                end
            else
                x = x.body
            end
            push!(wheres, var)
            io = IOContext(io, :unionall_env => var)
        end
        if x isa DataType
            show_datatype(io, x, wheres)
        else
            show(io, x)
        end
    end
    show_wheres(io, wheres)
end

# Check whether 'sym' (defined in module 'parent') is visible from module 'from'
# If an object with this name exists in 'from', we need to check that it's the same binding
# and that it's not deprecated.
function isvisible(sym::Symbol, parent::Module, from::Module)
    owner = ccall(:jl_binding_owner, Any, (Any, Any), parent, sym)
    from_owner = ccall(:jl_binding_owner, Any, (Any, Any), from, sym)
    return owner !== nothing && from_owner === owner &&
        !isdeprecated(parent, sym) &&
        isdefined(from, sym) # if we're going to return true, force binding resolution
end

function is_global_function(tn::Core.TypeName, globname::Union{Symbol,Nothing})
    if globname !== nothing
        globname_str = string(globname::Symbol)
        if ('#' ∉ globname_str && '@' ∉ globname_str && isdefined(tn, :module) &&
                isbindingresolved(tn.module, globname) && isdefined(tn.module, globname) &&
                isconcretetype(tn.wrapper) && isa(getfield(tn.module, globname), tn.wrapper))
            return true
        end
    end
    return false
end

function show_type_name(io::IO, tn::Core.TypeName)
    if tn === UnionAll.name
        # by coincidence, `typeof(Type)` is a valid representation of the UnionAll type.
        # intercept this case and print `UnionAll` instead.
        return print(io, "UnionAll")
    end
    globname = isdefined(tn, :mt) ? tn.mt.name : nothing
    globfunc = is_global_function(tn, globname)
    sym = (globfunc ? globname : tn.name)::Symbol
    globfunc && print(io, "typeof(")
    quo = false
    if !(get(io, :compact, false)::Bool)
        # Print module prefix unless type is visible from module passed to
        # IOContext If :module is not set, default to Main. nothing can be used
        # to force printing prefix
        from = get(io, :module, Main)
        if isdefined(tn, :module) && (from === nothing || !isvisible(sym, tn.module, from))
            show(io, tn.module)
            print(io, ".")
            if globfunc && !is_id_start_char(first(string(sym)))
                print(io, ':')
                if sym in quoted_syms
                    print(io, '(')
                    quo = true
                end
            end
        end
    end
    show_sym(io, sym)
    quo      && print(io, ")")
    globfunc && print(io, ")")
    nothing
end

function show_datatype(io::IO, x::DataType, wheres::Vector{TypeVar}=TypeVar[])
    parameters = x.parameters::SimpleVector
    istuple = x.name === Tuple.name
    n = length(parameters)

    # Print homogeneous tuples with more than 3 elements compactly as NTuple{N, T}
    if istuple
        if n > 3 && all(@nospecialize(i) -> (parameters[1] === i), parameters)
            print(io, "NTuple{", n, ", ")
            show(io, parameters[1])
            print(io, "}")
        else
            print(io, "Tuple{")
            # join(io, params, ", ") params but `show` it
            first = true
            for param in parameters
                first ? (first = false) : print(io, ", ")
                show(io, param)
            end
            print(io, "}")
        end
    else
        show_type_name(io, x.name)
        show_typeparams(io, parameters, (unwrap_unionall(x.name.wrapper)::DataType).parameters, wheres)
    end
end

function show_supertypes(io::IO, typ::DataType)
    print(io, typ)
    while typ != Any
        typ = supertype(typ)
        print(io, " <: ", typ)
    end
end

show_supertypes(typ::DataType) = show_supertypes(stdout, typ)

"""
    @show exs...

Prints one or more expressions, and their results, to `stdout`, and returns the last result.

See also: [`show`](@ref), [`@info`](@ref man-logging), [`println`](@ref).

# Examples
```jldoctest
julia> x = @show 1+2
1 + 2 = 3
3

julia> @show x^2 x/2;
x ^ 2 = 9
x / 2 = 1.5
```
"""
macro show(exs...)
    blk = Expr(:block)
    for ex in exs
        push!(blk.args, :(println($(sprint(show_unquoted,ex)*" = "),
                                  repr(begin local value = $(esc(ex)) end))))
    end
    isempty(exs) || push!(blk.args, :value)
    return blk
end

function show(io::IO, tn::Core.TypeName)
    print(io, "typename(")
    show_type_name(io, tn)
    print(io, ")")
end

show(io::IO, ::Nothing) = print(io, "nothing")
show(io::IO, b::Bool) = print(io, get(io, :typeinfo, Any) === Bool ? (b ? "1" : "0") : (b ? "true" : "false"))
show(io::IO, n::Signed) = (write(io, string(n)); nothing)
show(io::IO, n::Unsigned) = print(io, "0x", string(n, pad = sizeof(n)<<1, base = 16))
print(io::IO, n::Unsigned) = print(io, string(n))

show(io::IO, p::Ptr) = print(io, typeof(p), " @0x$(string(UInt(p), base = 16, pad = Sys.WORD_SIZE>>2))")

has_tight_type(p::Pair) =
    typeof(p.first)  == typeof(p).parameters[1] &&
    typeof(p.second) == typeof(p).parameters[2]

isdelimited(io::IO, x) = true
isdelimited(io::IO, x::Function) = !isoperator(Symbol(x))

# !isdelimited means that the Pair is printed with "=>" (like in "1 => 2"),
# without its explicit type (like in "Pair{Integer,Integer}(1, 2)")
isdelimited(io::IO, p::Pair) = !(has_tight_type(p) || get(io, :typeinfo, Any) == typeof(p))

function gettypeinfos(io::IO, p::Pair)
    typeinfo = get(io, :typeinfo, Any)
    p isa typeinfo <: Pair ?
        fieldtype(typeinfo, 1) => fieldtype(typeinfo, 2) :
        Any => Any
end

function show(io::IO, p::Pair)
    isdelimited(io, p) && return show_pairtyped(io, p)
    typeinfos = gettypeinfos(io, p)
    for i = (1, 2)
        io_i = IOContext(io, :typeinfo => typeinfos[i])
        isdelimited(io_i, p[i]) || print(io, "(")
        show(io_i, p[i])
        isdelimited(io_i, p[i]) || print(io, ")")
        i == 1 && print(io, get(io, :compact, false) ? "=>" : " => ")
    end
end

function show_pairtyped(io::IO, p::Pair{K,V}) where {K,V}
    show(io, typeof(p))
    show(io, (p.first, p.second))
end

function show(io::IO, m::Module)
    if is_root_module(m)
        print(io, nameof(m))
    else
        print_fullname(io, m)
    end
end
# The call to print_fullname above was originally `print(io, join(fullname(m),"."))`,
# which allocates. The method below provides the same behavior without allocating.
# See https://github.com/JuliaLang/julia/pull/42773 for perf information.
function print_fullname(io::IO, m::Module)
    mp = parentmodule(m)
    if m === Main || m === Base || m === Core || mp === m
        show_sym(io, nameof(m))
    else
        print_fullname(io, mp)
        print(io, '.')
        show_sym(io, nameof(m))
    end
end

function sourceinfo_slotnames(src::CodeInfo)
    slotnames = src.slotnames
    names = Dict{String,Int}()
    printnames = Vector{String}(undef, length(slotnames))
    for i in eachindex(slotnames)
        if slotnames[i] == :var"#unused#"
            printnames[i] = "_"
            continue
        end
        name = string(slotnames[i])
        idx = get!(names, name, i)
        if idx != i || isempty(name)
            printname = "$name@_$i"
            idx > 0 && (printnames[idx] = "$name@_$idx")
            names[name] = 0
        else
            printname = name
        end
        printnames[i] = printname
    end
    return printnames
end

show(io::IO, l::Core.MethodInstance) = show_mi(io, l)

function show_mi(io::IO, l::Core.MethodInstance, from_stackframe::Bool=false)
    def = l.def
    if isa(def, Method)
        if isdefined(def, :generator) && l === def.generator
            print(io, "MethodInstance generator for ")
            show(io, def)
        else
            print(io, "MethodInstance for ")
            show_tuple_as_call(io, def.name, l.specTypes; qualified=true)
        end
    else
        print(io, "Toplevel MethodInstance thunk")
        # `thunk` is not very much information to go on. If this
        # MethodInstance is part of a stacktrace, it gets location info
        # added by other means.  But if it isn't, then we should try
        # to print a little more identifying information.
        if !from_stackframe
            linetable = l.uninferred.linetable
            line = isempty(linetable) ? "unknown" : (lt = linetable[1]::Union{LineNumberNode,Core.LineInfoNode}; string(lt.file, ':', lt.line))
            print(io, " from ", def, " starting at ", line)
        end
    end
end

# These sometimes show up as Const-values in InferenceFrameInfo signatures
show(io::IO, r::Core.Compiler.UnitRange) = show(io, r.start : r.stop)
show(io::IO, mime::MIME{Symbol("text/plain")}, r::Core.Compiler.UnitRange) = show(io, mime, r.start : r.stop)

function show(io::IO, mi_info::Core.Compiler.Timings.InferenceFrameInfo)
    mi = mi_info.mi
    def = mi.def
    if isa(def, Method)
        if isdefined(def, :generator) && mi === def.generator
            print(io, "InferenceFrameInfo generator for ")
            show(io, def)
        else
            print(io, "InferenceFrameInfo for ")
            argnames = [isa(a, Core.Const) ? (isa(a.val, Type) ? "" : a.val) : "" for a in mi_info.slottypes[1:mi_info.nargs]]
            show_tuple_as_call(io, def.name, mi.specTypes; argnames, qualified=true)
        end
    else
        linetable = mi.uninferred.linetable
        line = isempty(linetable) ? "" : (lt = linetable[1]; string(lt.file, ':', lt.line))
        print(io, "Toplevel InferenceFrameInfo thunk from ", def, " starting at ", line)
    end
end

function show(io::IO, tinf::Core.Compiler.Timings.Timing)
    print(io, "Core.Compiler.Timings.Timing(", tinf.mi_info, ") with ", length(tinf.children), " children")
end

function show_delim_array(io::IO, itr::Union{AbstractArray,SimpleVector}, op, delim, cl,
                          delim_one, i1=first(LinearIndices(itr)), l=last(LinearIndices(itr)))
    print(io, op)
    if !show_circular(io, itr)
        recur_io = IOContext(io, :SHOWN_SET => itr)
        first = true
        i = i1
        if l >= i1
            while true
                if !isassigned(itr, i)
                    print(io, undef_ref_str)
                else
                    x = itr[i]
                    show(recur_io, x)
                end
                i += 1
                if i > l
                    delim_one && first && print(io, delim)
                    break
                end
                first = false
                print(io, delim)
                print(io, ' ')
            end
        end
    end
    print(io, cl)
end

function show_delim_array(io::IO, itr, op, delim, cl, delim_one, i1=1, n=typemax(Int))
    print(io, op)
    if !show_circular(io, itr)
        recur_io = IOContext(io, :SHOWN_SET => itr)
        y = iterate(itr)
        first = true
        i0 = i1-1
        while i1 > 1 && y !== nothing
            y = iterate(itr, y[2])
            i1 -= 1
        end
        if y !== nothing
            typeinfo = get(io, :typeinfo, Any)
            while true
                x = y[1]
                y = iterate(itr, y[2])
                show(IOContext(recur_io, :typeinfo => itr isa typeinfo <: Tuple ?
                                             fieldtype(typeinfo, i1+i0) :
                                             typeinfo),
                     x)
                i1 += 1
                if y === nothing || i1 > n
                    delim_one && first && print(io, delim)
                    break
                end
                first = false
                print(io, delim)
                print(io, ' ')
            end
        end
    end
    print(io, cl)
end

show(io::IO, t::Tuple) = show_delim_array(io, t, '(', ',', ')', true)
show(io::IO, v::SimpleVector) = show_delim_array(io, v, "svec(", ',', ')', false)

show(io::IO, s::Symbol) = show_unquoted_quote_expr(io, s, 0, 0, 0)

## Abstract Syntax Tree (AST) printing ##

# Summary:
#   print(io, ex) defers to show_unquoted(io, ex)
#   show(io, ex) defers to show_unquoted(io, QuoteNode(ex))
#   show_unquoted(io, ex) does the heavy lifting
#
# AST printing should follow two rules:
#   1. Meta.parse(string(ex)) == ex
#   2. eval(Meta.parse(repr(ex))) == ex
#
# Rule 1 means that printing an expression should generate Julia code which
# could be reparsed to obtain the original expression. This code should be
# unambiguous and as readable as possible.
#
# Rule 2 means that showing an expression should generate a quoted version of
# print’s output. Parsing and then evaling this output should return the
# original expression.
#
# This is consistent with many other show methods, i.e.:
#   show(Set([1,2,3]))                     # ==> "Set{Int64}([2,3,1])"
#   eval(Meta.parse("Set{Int64}([2,3,1])”) # ==> An actual set
# While this isn’t true of ALL show methods, it is of all ASTs.

const ExprNode = Union{Expr, QuoteNode, Slot, LineNumberNode, SSAValue,
                       GotoNode, GlobalRef, PhiNode, PhiCNode, UpsilonNode,
                       Core.Compiler.GotoIfNot, Core.Compiler.ReturnNode}
# Operators have precedence levels from 1-N, and show_unquoted defaults to a
# precedence level of 0 (the fourth argument). The top-level print and show
# methods use a precedence of -1 to specially allow space-separated macro syntax.
# IOContext(io, :unquote_fallback => false) tells show_unquoted to treat any
# Expr whose head is :$ as if it is inside a quote, preventing fallback to the
# "unhandled" case: this is used by print/string to be lawful to Rule 1 above.
# On the contrary, show/repr have to follow Rule 2, requiring any Expr whose
# head is :$ and which is not inside a quote to fallback to the "unhandled" case:
# this is behavior is triggered by IOContext(io, :unquote_fallback => true)
print(        io::IO, ex::ExprNode)    = (show_unquoted(IOContext(io, :unquote_fallback => false), ex, 0, -1); nothing)
show(         io::IO, ex::ExprNode)    = show_unquoted_quote_expr(IOContext(io, :unquote_fallback => true), ex, 0, -1, 0)
show_unquoted(io::IO, ex)              = show_unquoted(io, ex, 0, 0)
show_unquoted(io::IO, ex, indent::Int) = show_unquoted(io, ex, indent, 0)
show_unquoted(io::IO, ex, ::Int,::Int) = show(io, ex)
show_unquoted(io::IO, ex, indent::Int, prec::Int, ::Int) = show_unquoted(io, ex, indent, prec)

## AST printing constants ##

const indent_width = 4
const quoted_syms = Set{Symbol}([:(:),:(::),:(:=),:(=),:(==),:(===),:(=>)])
const uni_syms = Set{Symbol}([:(::), :(<:), :(>:)])
const uni_ops = Set{Symbol}([:(+), :(-), :(!), :(¬), :(~), :(<:), :(>:), :(√), :(∛), :(∜), :(∓), :(±)])
const expr_infix_wide = Set{Symbol}([
    :(=), :(+=), :(-=), :(*=), :(/=), :(\=), :(^=), :(&=), :(|=), :(÷=), :(%=), :(>>>=), :(>>=), :(<<=),
    :(.=), :(.+=), :(.-=), :(.*=), :(./=), :(.\=), :(.^=), :(.&=), :(.|=), :(.÷=), :(.%=), :(.>>>=), :(.>>=), :(.<<=),
    :(&&), :(||), :(<:), :($=), :(⊻=), :(>:), :(-->),
    :(:=), :(≔), :(⩴), :(≕)])
const expr_infix = Set{Symbol}([:(:), :(->), :(::)])
const expr_infix_any = union(expr_infix, expr_infix_wide)
const expr_calls  = Dict(:call => ('(',')'), :calldecl => ('(',')'),
                         :ref => ('[',']'), :curly => ('{','}'), :(.) => ('(',')'))
const expr_parens = Dict(:tuple=>('(',')'), :vcat=>('[',']'),
                         :hcat =>('[',']'), :row =>('[',']'), :vect=>('[',']'),
                         :ncat =>('[',']'), :nrow =>('[',']'),
                         :braces=>('{','}'), :bracescat=>('{','}'))

## AST decoding helpers ##

is_id_start_char(c::AbstractChar) = ccall(:jl_id_start_char, Cint, (UInt32,), c) != 0
is_id_char(c::AbstractChar) = ccall(:jl_id_char, Cint, (UInt32,), c) != 0

"""
     isidentifier(s) -> Bool

Return whether the symbol or string `s` contains characters that are parsed as
a valid ordinary identifier (not a binary/unary operator) in Julia code;
see also [`Base.isoperator`](@ref).

Internally Julia allows any sequence of characters in a `Symbol` (except `\\0`s),
and macros automatically use variable names containing `#` in order to avoid
naming collision with the surrounding code. In order for the parser to
recognize a variable, it uses a limited set of characters (greatly extended by
Unicode). `isidentifier()` makes it possible to query the parser directly
whether a symbol contains valid characters.

# Examples
```jldoctest
julia> Meta.isidentifier(:x), Meta.isidentifier("1x")
(true, false)
```
"""
function isidentifier(s::AbstractString)
    x = Iterators.peel(s)
    isnothing(x) && return false
    (s == "true" || s == "false") && return false
    c, rest = x
    is_id_start_char(c) || return false
    return all(is_id_char, rest)
end
isidentifier(s::Symbol) = isidentifier(string(s))

is_op_suffix_char(c::AbstractChar) = ccall(:jl_op_suffix_char, Cint, (UInt32,), c) != 0

_isoperator(s) = ccall(:jl_is_operator, Cint, (Cstring,), s) != 0

"""
    isoperator(s::Symbol)

Return `true` if the symbol can be used as an operator, `false` otherwise.

# Examples
```jldoctest
julia> Meta.isoperator(:+), Meta.isoperator(:f)
(true, false)
```
"""
isoperator(s::Union{Symbol,AbstractString}) = _isoperator(s) || ispostfixoperator(s)

"""
    isunaryoperator(s::Symbol)

Return `true` if the symbol can be used as a unary (prefix) operator, `false` otherwise.

# Examples
```jldoctest
julia> Meta.isunaryoperator(:-), Meta.isunaryoperator(:√), Meta.isunaryoperator(:f)
(true, true, false)
```
"""
isunaryoperator(s::Symbol) = ccall(:jl_is_unary_operator, Cint, (Cstring,), s) != 0
is_unary_and_binary_operator(s::Symbol) = ccall(:jl_is_unary_and_binary_operator, Cint, (Cstring,), s) != 0
is_syntactic_operator(s::Symbol) = ccall(:jl_is_syntactic_operator, Cint, (Cstring,), s) != 0

"""
    isbinaryoperator(s::Symbol)

Return `true` if the symbol can be used as a binary (infix) operator, `false` otherwise.

# Examples
```jldoctest
julia> Meta.isbinaryoperator(:-), Meta.isbinaryoperator(:√), Meta.isbinaryoperator(:f)
(true, false, false)
```
"""
function isbinaryoperator(s::Symbol)
    return _isoperator(s) && (!isunaryoperator(s) || is_unary_and_binary_operator(s)) &&
        s !== Symbol("'")
end

"""
    ispostfixoperator(s::Union{Symbol,AbstractString})

Return `true` if the symbol can be used as a postfix operator, `false` otherwise.

# Examples
```jldoctest
julia> Meta.ispostfixoperator(Symbol("'")), Meta.ispostfixoperator(Symbol("'ᵀ")), Meta.ispostfixoperator(:-)
(true, true, false)
```
"""
function ispostfixoperator(s::Union{Symbol,AbstractString})
    s = String(s)::String
    return startswith(s, '\'') && all(is_op_suffix_char, SubString(s, 2))
end

"""
    operator_precedence(s::Symbol)

Return an integer representing the precedence of operator `s`, relative to
other operators. Higher-numbered operators take precedence over lower-numbered
operators. Return `0` if `s` is not a valid operator.

# Examples
```jldoctest
julia> Base.operator_precedence(:+), Base.operator_precedence(:*), Base.operator_precedence(:.)
(11, 12, 17)

julia> Base.operator_precedence(:sin), Base.operator_precedence(:+=), Base.operator_precedence(:(=))  # (Note the necessary parens on `:(=)`)
(0, 1, 1)
```
"""
operator_precedence(s::Symbol) = Int(ccall(:jl_operator_precedence, Cint, (Cstring,), s))
operator_precedence(x::Any) = 0 # fallback for generic expression nodes
const prec_assignment = operator_precedence(:(=))
const prec_pair = operator_precedence(:(=>))
const prec_control_flow = operator_precedence(:(&&))
const prec_arrow = operator_precedence(:(-->))
const prec_comparison = operator_precedence(:(>))
const prec_power = operator_precedence(:(^))
const prec_decl = operator_precedence(:(::))

"""
    operator_associativity(s::Symbol)

Return a symbol representing the associativity of operator `s`. Left- and right-associative
operators return `:left` and `:right`, respectively. Return `:none` if `s` is non-associative
or an invalid operator.

# Examples
```jldoctest
julia> Base.operator_associativity(:-), Base.operator_associativity(:+), Base.operator_associativity(:^)
(:left, :none, :right)

julia> Base.operator_associativity(:⊗), Base.operator_associativity(:sin), Base.operator_associativity(:→)
(:left, :none, :right)
```
"""
function operator_associativity(s::Symbol)
    if operator_precedence(s) in (prec_arrow, prec_assignment, prec_control_flow, prec_pair, prec_power) ||
        (isunaryoperator(s) && !is_unary_and_binary_operator(s)) || s === :<| || s === :||
        return :right
    elseif operator_precedence(s) in (0, prec_comparison) || s in (:+, :++, :*)
        return :none
    end
    return :left
end

const is_expr = isexpr

is_quoted(ex)            = false
is_quoted(ex::QuoteNode) = true
is_quoted(ex::Expr)      = is_expr(ex, :quote, 1) || is_expr(ex, :inert, 1)

unquoted(ex::QuoteNode)  = ex.value
unquoted(ex::Expr)       = ex.args[1]

## AST printing helpers ##

function printstyled end
function with_output_color end

is_expected_union(u::Union) = u.a == Nothing || u.b == Nothing || u.a == Missing || u.b == Missing

emphasize(io, str::AbstractString, col = Base.error_color()) = get(io, :color, false) ?
    printstyled(io, str; color=col, bold=true) :
    print(io, uppercase(str))

show_linenumber(io::IO, line)       = printstyled(io, "#= line ", line, " =#", color=:light_black)
show_linenumber(io::IO, line, file) = printstyled(io, "#= ", file, ":", line, " =#", color=:light_black)
show_linenumber(io::IO, line, file::Nothing) = show_linenumber(io, line)

# show a block, e g if/for/etc
function show_block(io::IO, head, args::Vector, body, indent::Int, quote_level::Int)
    print(io, head)
    if !isempty(args)
        print(io, ' ')
        if head === :elseif
            show_list(io, args, " ", indent, 0, quote_level)
        else
            show_list(io, args, ", ", indent, 0, quote_level)
        end
    end

    ind = head === :module || head === :baremodule ? indent : indent + indent_width
    exs = (is_expr(body, :block) || is_expr(body, :quote)) ? body.args : Any[body]
    for ex in exs
        print(io, '\n', " "^ind)
        show_unquoted(io, ex, ind, -1, quote_level)
    end
    print(io, '\n', " "^indent)
end
show_block(io::IO,head,    block,i::Int, quote_level::Int) = show_block(io,head, [], block,i, quote_level)
function show_block(io::IO, head, arg, block, i::Int, quote_level::Int)
    if is_expr(arg, :block) || is_expr(arg, :quote)
        show_block(io, head, arg.args, block, i, quote_level)
    else
        show_block(io, head, Any[arg], block, i, quote_level)
    end
end

# show an indented list
function show_list(io::IO, items, sep, indent::Int, prec::Int=0, quote_level::Int=0, enclose_operators::Bool=false,
                   kw::Bool=false)
    n = length(items)
    n == 0 && return
    indent += indent_width
    first = true
    for item in items
        !first && print(io, sep)
        parens = !is_quoted(item) &&
            (first && prec >= prec_power &&
             ((item isa Expr && item.head === :call && (callee = item.args[1]; isa(callee, Symbol) && callee in uni_ops)) ||
              (item isa Real && item < 0))) ||
            (enclose_operators && item isa Symbol && isoperator(item) && is_valid_identifier(item))
        parens && print(io, '(')
        if kw && is_expr(item, :kw, 2)
            item = item::Expr
            show_unquoted(io, Expr(:(=), item.args[1], item.args[2]), indent, parens ? 0 : prec, quote_level)
        elseif kw && is_expr(item, :(=), 2)
            item = item::Expr
            show_unquoted_expr_fallback(io, item, indent, quote_level)
        else
            show_unquoted(io, item, indent, parens ? 0 : prec, quote_level)
        end
        parens && print(io, ')')
        first = false
    end
end
# show an indented list inside the parens (op, cl)
function show_enclosed_list(io::IO, op, items, sep, cl, indent, prec=0, quote_level=0, encl_ops=false, kw::Bool=false)
    print(io, op)
    show_list(io, items, sep, indent, prec, quote_level, encl_ops, kw)
    print(io, cl)
end

function is_valid_identifier(sym)
    return isidentifier(sym) || (
        _isoperator(sym) &&
        !(sym in (Symbol("'"), :(::), :?)) &&
        !is_syntactic_operator(sym)
    )
end

# show a normal (non-operator) function call, e.g. f(x, y) or A[z]
# kw: `=` expressions are parsed with head `kw` in this context
function show_call(io::IO, head, func, func_args, indent, quote_level, kw::Bool)
    op, cl = expr_calls[head]
    if (isa(func, Symbol) && func !== :(:) && !(head === :. && isoperator(func))) ||
            (isa(func, Symbol) && !is_valid_identifier(func)) ||
            (isa(func, Expr) && (func.head === :. || func.head === :curly || func.head === :macroname)) ||
            isa(func, GlobalRef)
        show_unquoted(io, func, indent, 0, quote_level)
    else
        print(io, '(')
        show_unquoted(io, func, indent, 0, quote_level)
        print(io, ')')
    end
    if head === :(.)
        print(io, '.')
    end
    if !isempty(func_args) && isa(func_args[1], Expr) && (func_args[1]::Expr).head === :parameters
        print(io, op)
        show_list(io, func_args[2:end], ", ", indent, 0, quote_level, false, kw)
        print(io, "; ")
        show_list(io, (func_args[1]::Expr).args, ", ", indent, 0, quote_level, false, kw)
        print(io, cl)
    else
        show_enclosed_list(io, op, func_args, ", ", cl, indent, 0, quote_level, false, kw)
    end
end

# Print `sym` as it would appear as an identifier name in code
# * Print valid identifiers & operators literally; also macros names if allow_macroname=true
# * Escape invalid identifiers with var"" syntax
function show_sym(io::IO, sym::Symbol; allow_macroname=false)
    if is_valid_identifier(sym)
        print(io, sym)
    elseif allow_macroname && (sym_str = string(sym); startswith(sym_str, '@'))
        print(io, '@')
        show_sym(io, Symbol(sym_str[2:end]))
    else
        print(io, "var", repr(string(sym)))
    end
end

## AST printing ##

show_unquoted(io::IO, val::SSAValue, ::Int, ::Int)      = print(io, "%", val.id)
show_unquoted(io::IO, sym::Symbol, ::Int, ::Int)        = show_sym(io, sym, allow_macroname=false)
show_unquoted(io::IO, ex::LineNumberNode, ::Int, ::Int) = show_linenumber(io, ex.line, ex.file)
show_unquoted(io::IO, ex::GotoNode, ::Int, ::Int)       = print(io, "goto %", ex.label)
show_unquoted(io::IO, ex::GlobalRef, ::Int, ::Int)      = show_globalref(io, ex)

function show_globalref(io::IO, ex::GlobalRef; allow_macroname=false)
    print(io, ex.mod)
    print(io, '.')
    quoted = !isidentifier(ex.name) && !startswith(string(ex.name), "@")
    parens = quoted && (!isoperator(ex.name) || (ex.name in quoted_syms))
    quoted && print(io, ':')
    parens && print(io, '(')
    show_sym(io, ex.name, allow_macroname=allow_macroname)
    parens && print(io, ')')
    nothing
end

function show_unquoted(io::IO, ex::Slot, ::Int, ::Int)
    typ = isa(ex, TypedSlot) ? ex.typ : Any
    slotid = ex.id
    slotnames = get(io, :SOURCE_SLOTNAMES, false)
    if (isa(slotnames, Vector{String}) &&
        slotid <= length(slotnames::Vector{String}))
        print(io, (slotnames::Vector{String})[slotid])
    else
        print(io, "_", slotid)
    end
    if typ !== Any && isa(ex, TypedSlot)
        print(io, "::", typ)
    end
end

function show_unquoted(io::IO, ex::QuoteNode, indent::Int, prec::Int)
    if isa(ex.value, Symbol)
        show_unquoted_quote_expr(io, ex.value, indent, prec, 0)
    else
        print(io, "\$(QuoteNode(")
        # QuoteNode does not allows for interpolation, so if ex.value is an
        # Expr it should be shown with quote_level equal to zero.
        # Calling show(io, ex.value) like this implicitly enforce that.
        show(io, ex.value)
        print(io, "))")
    end
end

function show_unquoted_quote_expr(io::IO, @nospecialize(value), indent::Int, prec::Int, quote_level::Int)
    if isa(value, Symbol)
        sym = value::Symbol
        if value in quoted_syms
            print(io, ":(", sym, ")")
        else
            if isidentifier(sym) || (_isoperator(sym) && sym !== Symbol("'"))
                print(io, ":", sym)
            else
                print(io, "Symbol(", repr(String(sym)), ")")
            end
        end
    else
        if isa(value,Expr) && value.head === :block
            value = value::Expr
            show_block(IOContext(io, beginsym=>false), "quote", value, indent, quote_level)
            print(io, "end")
        else
            print(io, ":(")
            show_unquoted(io, value, indent+2, -1, quote_level)  # +2 for `:(`
            print(io, ")")
        end
    end
end

function show_generator(io, ex::Expr, indent, quote_level)
    if ex.head === :flatten
        fg::Expr = ex
        ranges = Any[]
        while isa(fg, Expr) && fg.head === :flatten
            push!(ranges, (fg.args[1]::Expr).args[2:end])
            fg = (fg.args[1]::Expr).args[1]::Expr
        end
        push!(ranges, fg.args[2:end])
        show_unquoted(io, fg.args[1], indent, 0, quote_level)
        for r in ranges
            print(io, " for ")
            show_list(io, r, ", ", indent, 0, quote_level)
        end
    else
        show_unquoted(io, ex.args[1], indent, 0, quote_level)
        print(io, " for ")
        show_list(io, ex.args[2:end], ", ", indent, 0, quote_level)
    end
end

function valid_import_path(@nospecialize(ex), allow_as = true)
    if allow_as && is_expr(ex, :as) && length((ex::Expr).args) == 2
        ex = (ex::Expr).args[1]
    end
    return is_expr(ex, :(.)) && length((ex::Expr).args) > 0 && all(a->isa(a,Symbol), (ex::Expr).args)
end

function show_import_path(io::IO, ex, quote_level)
    if !isa(ex, Expr)
        show_unquoted(io, ex)
    elseif ex.head === :(:)
        show_import_path(io, ex.args[1], quote_level)
        print(io, ": ")
        for i = 2:length(ex.args)
            if i > 2
                print(io, ", ")
            end
            show_import_path(io, ex.args[i], quote_level)
        end
    elseif ex.head === :(.)
        for i = 1:length(ex.args)
            if ex.args[i] === :(.)
                print(io, '.')
            else
                show_sym(io, ex.args[i]::Symbol, allow_macroname=(i==length(ex.args)))
                i < length(ex.args) && print(io, '.')
            end
        end
    else
        show_unquoted(io, ex, 0, 0, quote_level)
    end
end

# Wrap symbols for macro names to allow them to be printed literally
function allow_macroname(ex)
    if (ex isa Symbol && first(string(ex)) == '@') ||
       ex isa GlobalRef ||
       (is_expr(ex, :(.)) && length(ex.args) == 2 &&
        (is_expr(ex.args[2], :quote) || ex.args[2] isa QuoteNode))
       return Expr(:macroname, ex)
    else
        ex
    end
end

function is_core_macro(arg, macro_name::AbstractString)
    arg === GlobalRef(Core, Symbol(macro_name))
end

# symbol for IOContext flag signaling whether "begin" is treated
# as an ordinary symbol, which is true in indexing expressions.
const beginsym = gensym(:beginsym)

function show_unquoted_expr_fallback(io::IO, ex::Expr, indent::Int, quote_level::Int)
    print(io, "\$(Expr(")
    show(io, ex.head)
    for arg in ex.args
        print(io, ", ")
        show(io, arg)
    end
    print(io, "))")
end

# TODO: implement interpolated strings
function show_unquoted(io::IO, ex::Expr, indent::Int, prec::Int, quote_level::Int = 0)
    head, args, nargs = ex.head, ex.args, length(ex.args)
    unhandled = false
    # dot (i.e. "x.y"), but not compact broadcast exps
    if head === :(.) && (nargs != 2 || !is_expr(args[2], :tuple))
        # standalone .op
        if nargs == 1 && args[1] isa Symbol && isoperator(args[1]::Symbol)
            print(io, "(.", args[1], ")")
        elseif nargs == 2 && is_quoted(args[2])
            item = args[1]
            # field
            field = unquoted(args[2])
            parens = !is_quoted(item) && !(item isa Symbol && isidentifier(item)) && !is_expr(item, :(.))
            parens && print(io, '(')
            show_unquoted(io, item, indent, 0, quote_level)
            parens && print(io, ')')
            # .
            print(io, '.')
            # item
            parens = !(field isa Symbol) || (field::Symbol in quoted_syms)
            quoted = parens || isoperator(field)
            quoted && print(io, ':')
            parens && print(io, '(')
            show_unquoted(io, field, indent, 0, quote_level)
            parens && print(io, ')')
        else
            unhandled = true
        end

    # infix (i.e. "x <: y" or "x = y")
    elseif (head in expr_infix_any && nargs==2)
        func_prec = operator_precedence(head)
        head_ = head in expr_infix_wide ? " $head " : head
        if func_prec <= prec
            show_enclosed_list(io, '(', args, head_, ')', indent, func_prec, quote_level, true)
        else
            show_list(io, args, head_, indent, func_prec, quote_level, true)
        end

    elseif head === :tuple
        print(io, "(")
        if nargs > 0 && is_expr(args[1], :parameters)
            arg1 = args[1]::Expr
            show_list(io, args[2:end], ", ", indent, 0, quote_level)
            nargs == 2 && print(io, ',')
            print(io, ";")
            if !isempty(arg1.args)
                print(io, " ")
            end
            show_list(io, arg1.args, ", ", indent, 0, quote_level, false, true)
        else
            show_list(io, args, ", ", indent, 0, quote_level)
            nargs == 1 && print(io, ',')
        end
        print(io, ")")

    # list-like forms, e.g. "[1, 2, 3]"
    elseif haskey(expr_parens, head) ||                          # :vcat etc.
        head === :typed_vcat || head === :typed_hcat || head === :typed_ncat
        # print the type and defer to the untyped case
        if head === :typed_vcat || head === :typed_hcat || head === :typed_ncat
            show_unquoted(io, args[1], indent, prec, quote_level)
            if head === :typed_vcat
                head = :vcat
            elseif head === :typed_hcat
                head = :hcat
            else
                head = :ncat
            end
            args = args[2:end]
            nargs = nargs - 1
        end
        op, cl = expr_parens[head]
        if head === :vcat || head === :bracescat
            sep = "; "
        elseif head === :hcat || head === :row
            sep = " "
        elseif head === :ncat || head === :nrow
            sep = ";"^args[1]::Int * " "
            args = args[2:end]
            nargs = nargs - 1
        else
            sep = ", "
        end
        head !== :row && head !== :nrow && print(io, op)
        show_list(io, args, sep, indent, 0, quote_level)
        if nargs <= 1 && (head === :vcat || head === :ncat)
            print(io, sep[1:end-1])
        end
        head !== :row && head !== :nrow && print(io, cl)

    # transpose
    elseif (head === Symbol("'") && nargs == 1) || (
        # ' with unicode suffix is a call expression
        head === :call && nargs == 2 && args[1] isa Symbol &&
        ispostfixoperator(args[1]::Symbol) && args[1]::Symbol !== Symbol("'")
    )
        op, arg1 = head === Symbol("'") ? (head, args[1]) : (args[1], args[2])
        if isa(arg1, Expr) || (isa(arg1, Symbol) && isoperator(arg1::Symbol))
            show_enclosed_list(io, '(', [arg1::Union{Expr, Symbol}], ", ", ')', indent, 0)
        else
            show_unquoted(io, arg1, indent, 0, quote_level)
        end
        print(io, op)

    # function call
    elseif head === :call && nargs >= 1
        func = args[1]
        fname = isa(func, GlobalRef) ? func.name : func
        func_prec = operator_precedence(fname)
        if func_prec > 0 || (isa(fname, Symbol) && fname in uni_ops)
            func = fname
        end
        func_args = args[2:end]

        # :kw exprs are only parsed inside parenthesized calls
        if any(a->is_expr(a, :kw), func_args) || (!isempty(func_args) && is_expr(func_args[1], :parameters))
            show_call(io, head, func, func_args, indent, quote_level, true)

        # scalar multiplication (i.e. "100x")
        elseif (func === :* &&
            length(func_args) == 2 && isa(func_args[1], Union{Int, Int64, Float32, Float64}) &&
            isa(func_args[2], Symbol) &&
            !in(string(func_args[2]::Symbol)[1], ('e', 'E', 'f', (func_args[1] == 0 && func_args[1] isa Integer ?
                                                                  # don't juxtapose 0 with b, o, x
                                                                  ('b', 'o', 'x') : ())...)))
            if func_prec <= prec
                show_enclosed_list(io, '(', func_args, "", ')', indent, func_prec, quote_level)
            else
                show_list(io, func_args, "", indent, func_prec, quote_level)
            end

        # unary operator (i.e. "!z")
        elseif isa(func,Symbol) && length(func_args) == 1 && func in uni_ops
            show_unquoted(io, func, indent, 0, quote_level)
            arg1 = func_args[1]
            if isa(arg1, Expr) || (isa(arg1, Symbol) && isoperator(arg1) && is_valid_identifier(arg1))
                show_enclosed_list(io, '(', func_args, ", ", ')', indent, func_prec)
            else
                show_unquoted(io, arg1, indent, func_prec, quote_level)
            end

        # binary operator (i.e. "x + y")
        elseif func_prec > 0 # is a binary operator
            na = length(func_args)
            if (na == 2 || (na > 2 && isa(func, Symbol) && func in (:+, :++, :*)) || (na == 3 && func === :(:))) &&
                    all(a -> !isa(a, Expr) || a.head !== :..., func_args)
                sep = func === :(:) ? "$func" : " " * convert(String, string(func))::String * " "   # if func::Any, avoid string interpolation (invalidation)

                if func_prec <= prec
                    show_enclosed_list(io, '(', func_args, sep, ')', indent, func_prec, quote_level, true)
                else
                    show_list(io, func_args, sep, indent, func_prec, quote_level, true)
                end
            elseif na == 1
                # 1-argument call to normally-binary operator
                op, cl = expr_calls[head]
                print(io, "(")
                show_unquoted(io, func, indent, 0, quote_level)
                print(io, ")")
                show_enclosed_list(io, op, func_args, ", ", cl, indent, 0, quote_level)
            else
                show_call(io, head, func, func_args, indent, quote_level, true)
            end

        # normal function (i.e. "f(x,y)")
        else
            show_call(io, head, func, func_args, indent, quote_level, true)
        end

    # new expr
    elseif head === :new || head === :splatnew
        show_enclosed_list(io, "%$head(", args, ", ", ")", indent, 0, quote_level)

    # other call-like expressions ("A[1,2]", "T{X,Y}", "f.(X,Y)")
    elseif haskey(expr_calls, head) && nargs >= 1  # :ref/:curly/:calldecl/:(.)
        funcargslike = head === :(.) ? (args[2]::Expr).args : args[2:end]
        show_call(head == :ref ? IOContext(io, beginsym=>true) : io, head, args[1], funcargslike, indent, quote_level, head !== :curly)

    # comprehensions
    elseif head === :typed_comprehension && nargs == 2
        show_unquoted(io, args[1], indent, 0, quote_level)
        print(io, '[')
        show_generator(io, args[2], indent, quote_level)
        print(io, ']')

    elseif head === :comprehension && nargs == 1
        print(io, '[')
        show_generator(io, args[1], indent, quote_level)
        print(io, ']')

    elseif (head === :generator && nargs >= 2) || (head === :flatten && nargs == 1)
        print(io, '(')
        show_generator(io, ex, indent, quote_level)
        print(io, ')')

    elseif head === :filter && nargs == 2
        show_unquoted(io, args[2], indent, 0, quote_level)
        print(io, " if ")
        show_unquoted(io, args[1], indent, 0, quote_level)

    # comparison (i.e. "x < y < z")
    elseif head === :comparison && nargs >= 3 && (nargs&1==1)
        comp_prec = minimum(operator_precedence, args[2:2:end])
        if comp_prec <= prec
            show_enclosed_list(io, '(', args, " ", ')', indent, comp_prec, quote_level)
        else
            show_list(io, args, " ", indent, comp_prec, quote_level)
        end

    # function calls need to transform the function from :call to :calldecl
    # so that operators are printed correctly
    elseif head === :function && nargs==2 && is_expr(args[1], :call)
        show_block(IOContext(io, beginsym=>false), head, Expr(:calldecl, (args[1]::Expr).args...), args[2], indent, quote_level)
        print(io, "end")

    elseif (head === :function || head === :macro) && nargs == 1
        print(io, head, ' ')
        show_unquoted(IOContext(io, beginsym=>false), args[1])
        print(io, " end")

    elseif head === :do && nargs == 2
        iob = IOContext(io, beginsym=>false)
        show_unquoted(iob, args[1], indent, -1, quote_level)
        print(io, " do ")
        show_list(iob, (((args[2]::Expr).args[1])::Expr).args, ", ", 0, 0, quote_level)
        for stmt in (((args[2]::Expr).args[2])::Expr).args
            print(io, '\n', " "^(indent + indent_width))
            show_unquoted(iob, stmt, indent + indent_width, -1, quote_level)
        end
        print(io, '\n', " "^indent)
        print(io, "end")

    # block with argument
    elseif head in (:for,:while,:function,:macro,:if,:elseif,:let) && nargs==2
        if is_expr(args[2], :block)
            show_block(IOContext(io, beginsym=>false), head, args[1], args[2], indent, quote_level)
        else
            show_block(IOContext(io, beginsym=>false), head, args[1], Expr(:block, args[2]), indent, quote_level)
        end
        print(io, "end")

    elseif (head === :if || head === :elseif) && nargs == 3
        iob = IOContext(io, beginsym=>false)
        show_block(iob, head, args[1], args[2], indent, quote_level)
        arg3 = args[3]
        if isa(arg3, Expr) && arg3.head === :elseif
            show_unquoted(iob, arg3::Expr, indent, prec, quote_level)
        else
            show_block(iob, "else", arg3, indent, quote_level)
            print(io, "end")
        end

    elseif head === :module && nargs==3 && isa(args[1],Bool)
        show_block(IOContext(io, beginsym=>false), args[1] ? :module : :baremodule, args[2], args[3], indent, quote_level)
        print(io, "end")

    # type declaration
    elseif head === :struct && nargs==3
        show_block(IOContext(io, beginsym=>false), args[1] ? Symbol("mutable struct") : Symbol("struct"), args[2], args[3], indent, quote_level)
        print(io, "end")

    elseif head === :primitive && nargs == 2
        print(io, "primitive type ")
        show_list(io, args, ' ', indent, 0, quote_level)
        print(io, " end")

    elseif head === :abstract && nargs == 1
        print(io, "abstract type ")
        show_list(IOContext(io, beginsym=>false), args, ' ', indent, 0, quote_level)
        print(io, " end")

    # empty return (i.e. "function f() return end")
    elseif head === :return && nargs == 1 && args[1] === nothing
        print(io, head)

    # type annotation (i.e. "::Int")
    elseif head in uni_syms && nargs == 1
        print(io, head)
        show_unquoted(io, args[1], indent, 0, quote_level)

    # var-arg declaration or expansion
    # (i.e. "function f(L...) end" or "f(B...)")
    elseif head === :(...) && nargs == 1
        dotsprec = operator_precedence(:(:)) - 1
        parens = dotsprec <= prec
        parens && print(io, "(")
        show_unquoted(io, args[1], indent, dotsprec, quote_level)
        print(io, "...")
        parens && print(io, ")")

    elseif (nargs == 0 && head in (:break, :continue))
        print(io, head)

    elseif (nargs == 1 && head in (:return, :const)) ||
                          head in (:local,  :global)
        print(io, head, ' ')
        show_list(io, args, ", ", indent, 0, quote_level)

    elseif head === :export
        print(io, head, ' ')
        show_list(io, mapany(allow_macroname, args), ", ", indent)

    elseif head === :macrocall && nargs >= 2
        # handle some special syntaxes
        # `a b c`
        if is_core_macro(args[1], "@cmd")
            print(io, "`", args[3], "`")
        # 11111111111111111111, 0xfffffffffffffffff, 1111...many digits...
        elseif is_core_macro(args[1], "@int128_str") ||
               is_core_macro(args[1], "@uint128_str") ||
               is_core_macro(args[1], "@big_str")
            print(io, args[3])
        # x"y" and x"y"z
        elseif isa(args[1], Symbol) && nargs >= 3 && isa(args[3], String) &&
               startswith(string(args[1]::Symbol), "@") &&
               endswith(string(args[1]::Symbol), "_str")
            s = string(args[1]::Symbol)
            print(io, s[2:prevind(s,end,4)], "\"")
            escape_raw_string(io, args[3])
            print(io, "\"")
            if nargs == 4
                print(io, args[4])
            end
        # general case
        else
            # first show the line number argument as a comment
            if isa(args[2], LineNumberNode) || is_expr(args[2], :line)
                print(io, args[2], ' ')
            end
            # Use the functional syntax unless specifically designated with
            # prec=-1 and hide the line number argument from the argument list
            mname = allow_macroname(args[1])
            if prec >= 0
                show_call(io, :call, mname, args[3:end], indent, quote_level, false)
            else
                show_args = Vector{Any}(undef, nargs - 1)
                show_args[1] = mname
                show_args[2:end] = args[3:end]
                show_list(io, show_args, ' ', indent, 0, quote_level)
            end
        end

    elseif head === :macroname && nargs == 1
        arg1 = args[1]
        if arg1 isa Symbol
            show_sym(io, arg1, allow_macroname=true)
        elseif arg1 isa GlobalRef
            show_globalref(io, arg1, allow_macroname=true)
        elseif is_expr(arg1, :(.)) && length((arg1::Expr).args) == 2
            arg1 = arg1::Expr
            m = arg1.args[1]
            if m isa Symbol || m isa GlobalRef || is_expr(m, :(.), 2)
                show_unquoted(io, m)
            else
                print(io, "(")
                show_unquoted(io, m)
                print(io, ")")
            end
            print(io, '.')
            if is_expr(arg1.args[2], :quote)
                mname = (arg1.args[2]::Expr).args[1]
            else
                mname = (arg1.args[2]::QuoteNode).value
            end
            if mname isa Symbol
                show_sym(io, mname, allow_macroname=true)
            else
                show_unquoted(io, mname)
            end
        else
            show_unquoted(io, arg1)
        end

    elseif head === :line && 1 <= nargs <= 2
        show_linenumber(io, args...)

    elseif head === :try && 3 <= nargs <= 5
        iob = IOContext(io, beginsym=>false)
        show_block(iob, "try", args[1], indent, quote_level)
        if is_expr(args[3], :block)
            show_block(iob, "catch", args[2] === false ? Any[] : args[2], args[3]::Expr, indent, quote_level)
        end
        if nargs >= 5 && is_expr(args[5], :block)
            show_block(iob, "else", Any[], args[5]::Expr, indent, quote_level)
        end
        if nargs >= 4 && is_expr(args[4], :block)
            show_block(iob, "finally", Any[], args[4]::Expr, indent, quote_level)
        end
        print(io, "end")

    elseif head === :block
        # print as (...; ...; ...;) inside indexing expression
        if get(io, beginsym, false)
            print(io, '(')
            ind = indent + indent_width
            for i = 1:length(ex.args)
                if i > 1
                    # if there was only a comment before the first semicolon, the expression would get parsed as a NamedTuple
                    if !(i == 2 && ex.args[1] isa LineNumberNode)
                        print(io, ';')
                    end
                    print(io, "\n", ' '^ind)
                end
                show_unquoted(io, ex.args[i], ind, -1, quote_level)
            end
            if length(ex.args) < 2
                print(io, isempty(ex.args) ? ";;)" : ";)")
            else
                print(io, ')')
            end
        else
            show_block(io, "begin", ex, indent, quote_level)
            print(io, "end")
        end

    elseif head === :quote && nargs == 1 && isa(args[1], Symbol)
        show_unquoted_quote_expr(IOContext(io, beginsym=>false), args[1]::Symbol, indent, 0, quote_level+1)
    elseif head === :quote && !(get(io, :unquote_fallback, true)::Bool)
        if nargs == 1 && is_expr(args[1], :block)
            show_block(IOContext(io, beginsym=>false), "quote", Expr(:quote, (args[1]::Expr).args...), indent,
                       quote_level+1)
            print(io, "end")
        elseif nargs == 1
            print(io, ":(")
            show_unquoted(IOContext(io, beginsym=>false), args[1], indent+2, 0, quote_level+1)
            print(io, ")")
        else
            show_block(IOContext(io, beginsym=>false), "quote", ex, indent, quote_level+1)
            print(io, "end")
        end

    elseif head === :gotoifnot && nargs == 2 && isa(args[2], Int)
        print(io, "unless ")
        show_unquoted(io, args[1], indent, 0, quote_level)
        print(io, " goto %")
        print(io, args[2]::Int)

    elseif head === :string && nargs == 1 && isa(args[1], AbstractString)
        show(io, args[1])

    elseif head === :null
        print(io, "nothing")

    elseif head === :string
        print(io, '"')
        for x in args
            if !isa(x,AbstractString)
                print(io, "\$(")
                if isa(x,Symbol) && !(x in quoted_syms)
                    show_sym(io, x)
                else
                    show_unquoted(io, x, 0, 0, quote_level)
                end
                print(io, ")")
            else
                escape_string(io, String(x)::String, "\"\$")
            end
        end
        print(io, '"')

    elseif (head === :& || head === :$) && nargs == 1
        if head === :$
            quote_level -= 1
        end
        if head === :$ && get(io, :unquote_fallback, true)
            unhandled = true
        else
            print(io, head)
            a1 = args[1]
            parens = (isa(a1,Expr) && !in(a1.head, (:tuple, :$, :vect, :braces))) ||
                     (isa(a1,Symbol) && isoperator(a1))
            parens && print(io, "(")
            show_unquoted(io, a1, 0, 0, quote_level)
            parens && print(io, ")")
        end

    # `where` syntax
    elseif head === :where && nargs > 1
        parens = 1 <= prec
        parens && print(io, "(")
        show_unquoted(io, args[1], indent, operator_precedence(:(::)), quote_level)
        print(io, " where ")
        if nargs == 2
            show_unquoted(io, args[2], indent, 1, quote_level)
        else
            print(io, "{")
            show_list(io, args[2:end], ", ", indent, 0, quote_level)
            print(io, "}")
        end
        parens && print(io, ")")

    elseif (head === :import || head === :using) &&
           ((nargs == 1 && (valid_import_path(args[1]) ||
                           (is_expr(args[1], :(:)) &&
                            length((args[1]::Expr).args) > 1 &&
                            all(valid_import_path, (args[1]::Expr).args)))) ||
             all(valid_import_path, args))
        print(io, head)
        print(io, ' ')
        first = true
        for a in args
            if !first
                print(io, ", ")
            end
            first = false
            show_import_path(io, a, quote_level)
        end
    elseif head === :as && nargs == 2 && valid_import_path(args[1], false)
        show_import_path(io, args[1], quote_level)
        print(io, " as ")
        show_unquoted(io, args[2], indent, 0, quote_level)
    elseif head === :meta && nargs >= 2 && args[1] === :push_loc
        print(io, "# meta: location ", join(args[2:end], " "))
    elseif head === :meta && nargs == 1 && args[1] === :pop_loc
        print(io, "# meta: pop location")
    elseif head === :meta && nargs == 2 && args[1] === :pop_loc
        print(io, "# meta: pop locations ($(args[2]::Int))")
    # print anything else as "Expr(head, args...)"
    else
        unhandled = true
    end
    if unhandled
        show_unquoted_expr_fallback(io, ex, indent, quote_level)
    end
    nothing
end

demangle_function_name(name::Symbol) = Symbol(demangle_function_name(string(name)))
function demangle_function_name(name::AbstractString)
    demangle = split(name, '#')
    # kw sorters and impl methods use the name scheme `f#...`
    if length(demangle) >= 2 && demangle[1] != ""
        return demangle[1]
    end
    return name
end

# show the called object in a signature, given its type `ft`
# `io` should contain the UnionAll env of the signature
function show_signature_function(io::IO, @nospecialize(ft), demangle=false, fargname="", html=false, qualified=false)
    uw = unwrap_unionall(ft)
    if ft <: Function && isa(uw, DataType) && isempty(uw.parameters) &&
        isdefined(uw.name.module, uw.name.mt.name) &&
        ft == typeof(getfield(uw.name.module, uw.name.mt.name))
        if qualified && !is_exported_from_stdlib(uw.name.mt.name, uw.name.module) && uw.name.module !== Main
            print_within_stacktrace(io, uw.name.module, '.', bold=true)
        end
        s = sprint(show_sym, (demangle ? demangle_function_name : identity)(uw.name.mt.name), context=io)
        print_within_stacktrace(io, s, bold=true)
    elseif isa(ft, DataType) && ft.name === Type.body.name &&
        (f = ft.parameters[1]; !isa(f, TypeVar))
        uwf = unwrap_unionall(f)
        parens = isa(f, UnionAll) && !(isa(uwf, DataType) && f === uwf.name.wrapper)
        parens && print(io, "(")
        print_within_stacktrace(io, f, bold=true)
        parens && print(io, ")")
    else
        if html
            print(io, "($fargname::<b>", ft, "</b>)")
        else
            print_within_stacktrace(io, "($fargname::", ft, ")", bold=true)
        end
    end
    nothing
end

function print_within_stacktrace(io, s...; color=:normal, bold=false)
    if get(io, :backtrace, false)::Bool
        printstyled(io, s...; color, bold)
    else
        print(io, s...)
    end
end

function show_tuple_as_call(io::IO, name::Symbol, sig::Type;
                            demangle=false, kwargs=nothing, argnames=nothing,
                            qualified=false, hasfirst=true)
    # print a method signature tuple for a lambda definition
    if sig === Tuple
        print(io, demangle ? demangle_function_name(name) : name, "(...)")
        return
    end
    tv = Any[]
    env_io = io
    while isa(sig, UnionAll)
        push!(tv, sig.var)
        env_io = IOContext(env_io, :unionall_env => sig.var)
        sig = sig.body
    end
    n = 1
    sig = (sig::DataType).parameters
    if hasfirst
        show_signature_function(env_io, sig[1], demangle, "", false, qualified)
        n += 1
    end
    first = true
    print_within_stacktrace(io, "(", bold=true)
    show_argnames = argnames !== nothing && length(argnames) == length(sig)
    for i = n:length(sig)  # fixme (iter): `eachindex` with offset?
        first || print(io, ", ")
        first = false
        if show_argnames
            print_within_stacktrace(io, argnames[i]; color=:light_black)
        end
        print(io, "::")
        print_type_stacktrace(env_io, sig[i])
    end
    if kwargs !== nothing
        print(io, "; ")
        first = true
        for (k, t) in kwargs
            first || print(io, ", ")
            first = false
            print_within_stacktrace(io, k; color=:light_black)
            print(io, "::")
            print_type_stacktrace(io, t)
        end
    end
    print_within_stacktrace(io, ")", bold=true)
    show_method_params(io, tv)
    nothing
end

function print_type_stacktrace(io, type; color=:normal)
    str = sprint(show, type, context=io)
    i = findfirst('{', str)
    if !get(io, :backtrace, false)::Bool
        print(io, str)
    elseif i === nothing
        printstyled(io, str; color=color)
    else
        printstyled(io, str[1:prevind(str,i)]; color=color)
        printstyled(io, str[i:end]; color=:light_black)
    end
end

resolvebinding(@nospecialize(ex)) = ex
resolvebinding(ex::QuoteNode) = ex.value
resolvebinding(ex::Symbol) = resolvebinding(GlobalRef(Main, ex))
function resolvebinding(ex::Expr)
    if ex.head === :. && isa(ex.args[2], Symbol)
        parent = resolvebinding(ex.args[1])
        if isa(parent, Module)
            return resolvebinding(GlobalRef(parent, ex.args[2]))
        end
    end
    return nothing
end
function resolvebinding(ex::GlobalRef)
    isdefined(ex.mod, ex.name) || return nothing
    isconst(ex.mod, ex.name) || return nothing
    m = getfield(ex.mod, ex.name)
    isa(m, Module) || return nothing
    return m
end

function ismodulecall(ex::Expr)
    return ex.head === :call && (ex.args[1] === GlobalRef(Base,:getfield) ||
                                ex.args[1] === GlobalRef(Core,:getfield)) &&
           isa(resolvebinding(ex.args[2]), Module)
end

function show(io::IO, tv::TypeVar)
    # If we are in the `unionall_env`, the type-variable is bound
    # and the type constraints are already printed.
    # We don't need to print it again.
    # Otherwise, the lower bound should be printed if it is not `Bottom`
    # and the upper bound should be printed if it is not `Any`.
    in_env = (:unionall_env => tv) in io
    function show_bound(io::IO, @nospecialize(b))
        parens = isa(b,UnionAll) && !print_without_params(b)
        parens && print(io, "(")
        show(io, b)
        parens && print(io, ")")
    end
    lb, ub = tv.lb, tv.ub
    if !in_env && lb !== Bottom
        if ub === Any
            show_unquoted(io, tv.name)
            print(io, ">:")
            show_bound(io, lb)
        else
            show_bound(io, lb)
            print(io, "<:")
            show_unquoted(io, tv.name)
        end
    else
        show_unquoted(io, tv.name)
    end
    if !in_env && ub !== Any
        print(io, "<:")
        show_bound(io, ub)
    end
    nothing
end

function show(io::IO, vm::Core.TypeofVararg)
    print(io, "Vararg")
    if isdefined(vm, :T)
        print(io, "{")
        show(io, vm.T)
        if isdefined(vm, :N)
            print(io, ", ")
            show(io, vm.N)
        end
        print(io, "}")
    end
end

module IRShow
    const Compiler = Core.Compiler
    using Core.IR
    import ..Base
    import .Compiler: IRCode, ReturnNode, GotoIfNot, CFG, scan_ssa_use!, Argument,
        isexpr, compute_basic_blocks, block_for_inst,
        TriState, Effects, ALWAYS_TRUE, ALWAYS_FALSE, TRISTATE_UNKNOWN
    Base.getindex(r::Compiler.StmtRange, ind::Integer) = Compiler.getindex(r, ind)
    Base.size(r::Compiler.StmtRange) = Compiler.size(r)
    Base.first(r::Compiler.StmtRange) = Compiler.first(r)
    Base.last(r::Compiler.StmtRange) = Compiler.last(r)
    Base.length(is::Compiler.InstructionStream) = Compiler.length(is)
    Base.iterate(is::Compiler.InstructionStream, st::Int=1) = (st <= Compiler.length(is)) ? (is[st], st + 1) : nothing
    Base.getindex(is::Compiler.InstructionStream, idx::Int) = Compiler.getindex(is, idx)
    Base.getindex(node::Compiler.Instruction, fld::Symbol) = Compiler.getindex(node, fld)
    include("compiler/ssair/show.jl")

    const __debuginfo = Dict{Symbol, Any}(
        # :full => src -> Base.IRShow.statementidx_lineinfo_printer(src), # and add variable slot information
        :source => src -> Base.IRShow.statementidx_lineinfo_printer(src),
        # :oneliner => src -> Base.IRShow.statementidx_lineinfo_printer(Base.IRShow.PartialLineInfoPrinter, src),
        :none => src -> Base.IRShow.lineinfo_disabled,
        )
    const default_debuginfo = Ref{Symbol}(:none)
    debuginfo(sym) = sym === :default ? default_debuginfo[] : sym
end

function show(io::IO, src::CodeInfo; debuginfo::Symbol=:source)
    # Fix slot names and types in function body
    print(io, "CodeInfo(")
    lambda_io::IOContext = io
    if src.slotnames !== nothing
        lambda_io = IOContext(lambda_io, :SOURCE_SLOTNAMES => sourceinfo_slotnames(src))
    end
    if isempty(src.linetable) || src.linetable[1] isa LineInfoNode
        println(io)
        # TODO: static parameter values?
        # only accepts :source or :none, we can't have a fallback for default since
        # that would break code_typed(, debuginfo=:source) iff IRShow.default_debuginfo[] = :none
        IRShow.show_ir(lambda_io, src, IRShow.IRShowConfig(IRShow.__debuginfo[debuginfo](src)))
    else
        # this is a CodeInfo that has not been used as a method yet, so its locations are still LineNumberNodes
        body = Expr(:block)
        body.args = src.code
        show(lambda_io, body)
    end
    print(io, ")")
end

function show(io::IO, inferred::Core.Compiler.InferenceResult)
    tt = inferred.linfo.specTypes.parameters[2:end]
    tts = join(["::$(t)" for t in tt], ", ")
    rettype = inferred.result
    if isa(rettype, Core.Compiler.InferenceState)
        rettype = rettype.bestguess
    end
    print(io, "$(inferred.linfo.def.name)($(tts)) => $(rettype)")
end

function show(io::IO, ::Core.Compiler.NativeInterpreter)
    print(io, "Core.Compiler.NativeInterpreter(...)")
end


function dump(io::IOContext, x::SimpleVector, n::Int, indent)
    if isempty(x)
        print(io, "empty SimpleVector")
        return
    end
    print(io, "SimpleVector")
    if n > 0
        for i = 1:length(x)
            println(io)
            print(io, indent, "  ", i, ": ")
            if isassigned(x,i)
                dump(io, x[i], n - 1, string(indent, "  "))
            else
                print(io, undef_ref_str)
            end
        end
    end
    nothing
end

function dump(io::IOContext, @nospecialize(x), n::Int, indent)
    if x === Union{}
        show(io, x)
        return
    end
    T = typeof(x)
    if isa(x, Function)
        print(io, x, " (function of type ", T, ")")
    else
        print(io, T)
    end
    nf = nfields(x)
    if nf > 0
        if n > 0 && !show_circular(io, x)
            recur_io = IOContext(io, Pair{Symbol,Any}(:SHOWN_SET, x))
            for field in 1:nf
                println(io)
                fname = string(fieldname(T, field))
                print(io, indent, "  ", fname, ": ")
                if isdefined(x,field)
                    dump(recur_io, getfield(x, field), n - 1, string(indent, "  "))
                else
                    print(io, undef_ref_str)
                end
            end
        end
    elseif !isa(x, Function)
        print(io, " ")
        show(io, x)
    end
    nothing
end

dump(io::IOContext, x::Module, n::Int, indent) = print(io, "Module ", x)
dump(io::IOContext, x::String, n::Int, indent) = (print(io, "String "); show(io, x))
dump(io::IOContext, x::Symbol, n::Int, indent) = print(io, typeof(x), " ", x)
dump(io::IOContext, x::Union,  n::Int, indent) = print(io, x)
dump(io::IOContext, x::Ptr,    n::Int, indent) = print(io, x)

function dump_elts(io::IOContext, x::Array, n::Int, indent, i0, i1)
    for i in i0:i1
        print(io, indent, "  ", i, ": ")
        if !isassigned(x,i)
            print(io, undef_ref_str)
        else
            dump(io, x[i], n - 1, string(indent, "  "))
        end
        i < i1 && println(io)
    end
end

function dump(io::IOContext, x::Array, n::Int, indent)
    print(io, "Array{", eltype(x), "}(", size(x), ")")
    if eltype(x) <: Number
        print(io, " ")
        show(io, x)
    else
        if n > 0 && !isempty(x) && !show_circular(io, x)
            println(io)
            recur_io = IOContext(io, :SHOWN_SET => x)
            lx = length(x)
            if get(io, :limit, false)
                dump_elts(recur_io, x, n, indent, 1, (lx <= 10 ? lx : 5))
                if lx > 10
                    println(io)
                    println(io, indent, "  ...")
                    dump_elts(recur_io, x, n, indent, lx - 4, lx)
                end
            else
                dump_elts(recur_io, x, n, indent, 1, lx)
            end
        end
    end
    nothing
end

# Types
function dump(io::IOContext, x::DataType, n::Int, indent)
    print(io, x)
    if x !== Any
        print(io, " <: ", supertype(x))
    end
    if n > 0 && !(x <: Tuple) && !isabstracttype(x)
        tvar_io::IOContext = io
        for tparam in x.parameters
            # approximately recapture the list of tvar parameterization
            # that may be used by the internal fields
            if isa(tparam, TypeVar)
                tvar_io = IOContext(tvar_io, :unionall_env => tparam)
            end
        end
        if x.name === NamedTuple_typename && !(x.parameters[1] isa Tuple)
            # named tuple type with unknown field names
            return
        end
        fields = fieldnames(x)
        fieldtypes = datatype_fieldtypes(x)
        for idx in 1:length(fields)
            println(io)
            print(io, indent, "  ", fields[idx], "::")
            print(tvar_io, fieldtypes[idx])
        end
    end
    nothing
end

const DUMP_DEFAULT_MAXDEPTH = 8

function dump(io::IO, @nospecialize(x); maxdepth=DUMP_DEFAULT_MAXDEPTH)
    dump(IOContext(io), x, maxdepth, "")
    println(io)
end

"""
    dump(x; maxdepth=$DUMP_DEFAULT_MAXDEPTH)

Show every part of the representation of a value.
The depth of the output is truncated at `maxdepth`.

# Examples
```jldoctest
julia> struct MyStruct
           x
           y
       end

julia> x = MyStruct(1, (2,3));

julia> dump(x)
MyStruct
  x: Int64 1
  y: Tuple{Int64, Int64}
    1: Int64 2
    2: Int64 3

julia> dump(x; maxdepth = 1)
MyStruct
  x: Int64 1
  y: Tuple{Int64, Int64}
```
"""
function dump(arg; maxdepth=DUMP_DEFAULT_MAXDEPTH)
    # this is typically used interactively, so default to being in Main
    mod = get(stdout, :module, Main)
    dump(IOContext(stdout, :limit => true, :module => mod), arg; maxdepth=maxdepth)
end

nocolor(io::IO) = IOContext(io, :color => false)
alignment_from_show(io::IO, x::Any) =
    textwidth(sprint(show, x, context=nocolor(io), sizehint=0))

"""
`alignment(io, X)` returns a tuple (left,right) showing how many characters are
needed on either side of an alignment feature such as a decimal point.

# Examples
```jldoctest
julia> Base.alignment(stdout, 42)
(2, 0)

julia> Base.alignment(stdout, 4.23)
(1, 3)

julia> Base.alignment(stdout, 1 + 10im)
(3, 5)
```
"""
alignment(io::IO, x::Any) = (0, alignment_from_show(io, x))
alignment(io::IO, x::Number) = (alignment_from_show(io, x), 0)
alignment(io::IO, x::Integer) = (alignment_from_show(io, x), 0)
function alignment(io::IO, x::Real)
    s = sprint(show, x, context=nocolor(io), sizehint=0)
    m = match(r"^(.*?)((?:[\.eEfF].*)?)$", s)
    m === nothing ? (textwidth(s), 0) :
                    (textwidth(m.captures[1]), textwidth(m.captures[2]))
end
function alignment(io::IO, x::Complex)
    s = sprint(show, x, context=nocolor(io), sizehint=0)
    m = match(r"^(.*[^ef][\+\-])(.*)$", s)
    m === nothing ? (textwidth(s), 0) :
                    (textwidth(m.captures[1]), textwidth(m.captures[2]))
end
function alignment(io::IO, x::Rational)
    s = sprint(show, x, context=nocolor(io), sizehint=0)
    m = match(r"^(.*?/)(/.*)$", s)
    m === nothing ? (textwidth(s), 0) :
                    (textwidth(m.captures[1]), textwidth(m.captures[2]))
end

function alignment(io::IO, x::Pair)
    fullwidth = alignment_from_show(io, x)
    if !isdelimited(io, x) # i.e. use "=>" for display
        ctx = IOContext(io, :typeinfo => gettypeinfos(io, x)[1])
        left = alignment_from_show(ctx, x.first)
        left += 2 * !isdelimited(ctx, x.first) # for parens around p.first
        left += !(get(io, :compact, false)::Bool) # spaces are added around "=>"
        (left+1, fullwidth-left-1) # +1 for the "=" part of "=>"
    else
        (0, fullwidth) # as for x::Any
    end
end

const undef_ref_str = "#undef"

show(io::IO, ::UndefInitializer) = print(io, "UndefInitializer()")

"""
    summary(io::IO, x)
    str = summary(x)

Print to a stream `io`, or return a string `str`, giving a brief description of
a value. By default returns `string(typeof(x))`, e.g. [`Int64`](@ref).

For arrays, returns a string of size and type info,
e.g. `10-element Array{Int64,1}`.

# Examples
```jldoctest
julia> summary(1)
"Int64"

julia> summary(zeros(2))
"2-element Vector{Float64}"
```
"""
summary(io::IO, x) = print(io, typeof(x))
function summary(x)
    io = IOBuffer()
    summary(io, x)
    String(take!(io))
end

## `summary` for AbstractArrays
# sizes such as 0-dimensional, 4-dimensional, 2x3
dims2string(d) = isempty(d) ? "0-dimensional" :
                 length(d) == 1 ? "$(d[1])-element" :
                 join(map(string,d), '×')

inds2string(inds) = join(map(_indsstring,inds), '×')
_indsstring(i) = string(i)
_indsstring(i::Union{IdentityUnitRange, Slice}) = string(i.indices)

# anything array-like gets summarized e.g. 10-element Array{Int64,1}
summary(io::IO, a::AbstractArray) = array_summary(io, a, axes(a))
function array_summary(io::IO, a, inds::Tuple{Vararg{OneTo}})
    print(io, dims2string(length.(inds)), " ")
    showarg(io, a, true)
end
function array_summary(io::IO, a, inds)
    print(io, dims2string(length.(inds)), " ")
    showarg(io, a, true)
    print(io, " with indices ", inds2string(inds))
end

## `summary` for Function
summary(io::IO, f::Function) = show(io, MIME"text/plain"(), f)

"""
    showarg(io::IO, x, toplevel)

Show `x` as if it were an argument to a function. This function is
used by [`summary`](@ref) to display type information in terms of sequences of
function calls on objects. `toplevel` is `true` if this is
the direct call from `summary` and `false` for nested (recursive) calls.

The fallback definition is to print `x` as "::\\\$(typeof(x))",
representing argument `x` in terms of its type. (The double-colon is
omitted if `toplevel=true`.) However, you can
specialize this function for specific types to customize printing.

# Example

A SubArray created as `view(a, :, 3, 2:5)`, where `a` is a
3-dimensional Float64 array, has type

    SubArray{Float64, 2, Array{Float64, 3}, Tuple{Colon, Int64, UnitRange{Int64}}, false}

The default `show` printing would display this full type.
However, the summary for SubArrays actually prints as

    2×4 view(::Array{Float64, 3}, :, 3, 2:5) with eltype Float64

because of a definition similar to

    function Base.showarg(io::IO, v::SubArray, toplevel)
        print(io, "view(")
        showarg(io, parent(v), false)
        print(io, ", ", join(v.indices, ", "))
        print(io, ')')
        toplevel && print(io, " with eltype ", eltype(v))
    end

Note that we're calling `showarg` recursively for the parent array
type, indicating that any recursed calls are not at the top level.
Printing the parent as `::Array{Float64,3}` is the fallback (non-toplevel)
behavior, because no specialized method for `Array` has been defined.
"""
function showarg(io::IO, T::Type, toplevel)
    toplevel || print(io, "::")
    print(io, "Type{", T, "}")
end
function showarg(io::IO, @nospecialize(x), toplevel)
    toplevel || print(io, "::")
    print(io, typeof(x))
end
# This method resolves an ambiguity for packages that specialize on eltype
function showarg(io::IO, a::Array{Union{}}, toplevel)
    toplevel || print(io, "::")
    print(io, typeof(a))
end

# Container specializations
function showarg(io::IO, v::SubArray, toplevel)
    print(io, "view(")
    showarg(io, parent(v), false)
    showindices(io, v.indices...)
    print(io, ')')
    toplevel && print(io, " with eltype ", eltype(v))
    return nothing
end
showindices(io, ::Slice, inds...) =
    (print(io, ", :"); showindices(io, inds...))
showindices(io, ind1, inds...) =
    (print(io, ", ", ind1); showindices(io, inds...))
showindices(io) = nothing

function showarg(io::IO, r::ReshapedArray, toplevel)
    print(io, "reshape(")
    showarg(io, parent(r), false)
    print(io, ", ", join(r.dims, ", "))
    print(io, ')')
    toplevel && print(io, " with eltype ", eltype(r))
    return nothing
end

function showarg(io::IO, r::NonReshapedReinterpretArray{T}, toplevel) where {T}
    print(io, "reinterpret(", T, ", ")
    showarg(io, parent(r), false)
    print(io, ')')
end

function showarg(io::IO, r::ReshapedReinterpretArray{T}, toplevel) where {T}
    print(io, "reinterpret(reshape, ", T, ", ")
    showarg(io, parent(r), false)
    print(io, ')')
    toplevel && print(io, " with eltype ", eltype(r))
    return nothing
end

# printing iterators from Base.Iterators

function show(io::IO, e::Iterators.Enumerate)
    print(io, "enumerate(")
    show(io, e.itr)
    print(io, ')')
end
show(io::IO, z::Iterators.Zip) = show_delim_array(io, z.is, "zip(", ',', ')', false)

# pretty printing for Iterators.Pairs
function Base.showarg(io::IO, r::Iterators.Pairs{<:Integer, <:Any, <:Any, T}, toplevel) where T<:AbstractArray
    print(io, "pairs(IndexLinear(), ::", T, ")")
end

function Base.showarg(io::IO, r::Iterators.Pairs{Symbol, <:Any, <:Any, T}, toplevel) where {T <: NamedTuple}
    print(io, "pairs(::NamedTuple)")
end

function Base.showarg(io::IO, r::Iterators.Pairs{<:Any, <:Any, I, D}, toplevel) where {D, I}
    print(io, "Iterators.Pairs(::", D, ", ::", I, ")")
end

# printing BitArrays

# (following functions not exported - mainly intended for debug)

function print_bit_chunk(io::IO, c::UInt64, l::Integer = 64)
    for s = 0:l-1
        d = (c >>> s) & 1
        print(io, "01"[d + 1])
        if (s + 1) & 7 == 0
            print(io, " ")
        end
    end
end

print_bit_chunk(c::UInt64, l::Integer) = print_bit_chunk(stdout, c, l)
print_bit_chunk(c::UInt64) = print_bit_chunk(stdout, c)

function bitshow(io::IO, B::BitArray)
    isempty(B) && return
    Bc = B.chunks
    for i = 1:length(Bc)-1
        print_bit_chunk(io, Bc[i])
        print(io, ": ")
    end
    l = _mod64(length(B)-1) + 1
    print_bit_chunk(io, Bc[end], l)
end
bitshow(B::BitArray) = bitshow(stdout, B)

bitstring(B::BitArray) = sprint(bitshow, B)

# printing OpaqueClosure
function show(io::IO, oc::Core.OpaqueClosure)
    A, R = typeof(oc).parameters
    show_tuple_as_call(io, Symbol(""), A; hasfirst=false)
    print(io, "::", R)
    print(io, "->◌")
end

function show(io::IO, ::MIME"text/plain", oc::Core.OpaqueClosure{A, R}) where {A, R}
    show(io, oc)
end
