│ ─ %-1  = invoke foo(::Int64,::Int64)::Union{}
    [33m @ foobar.jl:2 within `foo`[39m
    [33m┌ @ boot.jl: within `Box`[39m
[90m1 ──[39m[33m│[39m %1  = Core.Box[36m::Core.Const(Core.Box)[39m
[90m│   [39m[33m│[39m %2  = %new(%1)[36m::Core.Box[39m
[90m│ [39m  [33m└[39m
[90m│ [39m  [33m┌ @ int.jl: within `+`[39m
[90m│   [39m[33m│[39m %3  = Base.add_int(_2, _3)[36m::Int64[39m
[90m│ [39m  [33m└[39m
[90m└───[39m       Core.setfield!(%2, :contents, %3)[90m::Int64[39m
    [33m @ foobar.jl:3 within `foo`[39m
[90m2 ┄─[39m %5  = Core.isdefined(%2, :contents)[36m::Bool[39m
[90m└───[39m       goto #4 if not %5
[90m3 ──[39m       goto #5
[90m4 ──[39m       $(Expr(:throw_undef_if_not, :z, false))[90m::Any[39m
[90m5 ┄─[39m %9  = Core.getfield(%2, :contents)[36m::Any[39m
[90m│   [39m %10 = (%9 < 4)[36m::Any[39m
[90m└───[39m       goto #10 if not %10
    [33m @ foobar.jl:4 within `foo`[39m
[90m6 ──[39m %12 = Core.isdefined(%2, :contents)[36m::Bool[39m
[90m└───[39m       goto #8 if not %12
[90m7 ──[39m       goto #9
[90m8 ──[39m       $(Expr(:throw_undef_if_not, :z, false))[90m::Any[39m
[90m9 ┄─[39m %16 = Core.getfield(%2, :contents)[36m::Any[39m
[90m│   [39m %17 = (%16 + 1)[36m::Any[39m
[90m│   [39m       Core.setfield!(%2, :contents, %17)[90m::Any[39m
[90m└───[39m       goto #2
    [33m @ foobar.jl:6 within `foo`[39m
[90m10 ─[39m %20 = %new(Main.anonymous.:(var"#1#2"), %2)[36m::Main.anonymous.var"#1#2"[39m
[90m│   [39m       invoke %20(_2::Int64)[90m::Any[39m
[90m│ [39m  [33m @ foobar.jl:7 within `foo`[39m
[90m│ [39m  [33m┌ @ refpointer.jl: within `Ref`[39m
[90m│ [39m  [33m│┌ @ refvalue.jl: within `RefValue`[39m
[90m│   [39m[33m││[39m %22 = %new(Base.RefValue{Union{Missing, Int64}}, _2)[36m::Base.RefValue{Union{Missing, Int64}}[39m
[90m│ [39m  [33m└└[39m
[90m│   [39m       (%22 + _3)[90m::Union{}[39m
[90m└───[39m       unreachable
