UndefVarError when trying to use parametric function expression

[nsajko]

No replies on discourse, so I guess this indeed is a bug.

Suppose we have this:

SomeType = Tuple{Int8, Int16}
f(::Type{Tuple{A, B}}) where {A <: Any, B <: Any} = B

If we now do f(SomeType), we get Int16 in return as expected.

The following however, results in ERROR: UndefVarError: B not defined:

(function(::Type{Tuple{A, B}}) where {A <: Any, B <: Any}
   B
 end)(SomeType)

versioninfo() output: (1.7.3 behaves the same in this case)

Julia Version 1.8.0-beta3
Commit 3e092a2521 (2022-03-29 15:42 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: 8 × AMD Ryzen 3 5300U with Radeon Graphics
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-13.0.1 (ORCJIT, znver2)
  Threads: 1 on 8 virtual cores
Environment:
  LD_PRELOAD = /usr/lib/libswmhack.so.0.0

[vtjnash]

more concisely, we have just the anonymous definition throwing the error, even without attempting to call it:

julia> function(::Type{Tuple{A, B}}) where {A, B} end
ERROR: UndefVarError: B not defined

lowering has forgotten about B quite completely

julia> Meta.@lower function(::Type{Tuple{A, B}}) where {A, B} end
:($(Expr(:thunk, CodeInfo(
    @ none within `top-level scope`
1 ─      $(Expr(:thunk, CodeInfo(
    @ none within `top-level scope`
1 ─      global var"#25#26"
│        const var"#25#26"
│   %3 = Core._structtype(Main, Symbol("#25#26"), Core.svec(), Core.svec(), Core.svec(), false, 0)
│        Core._setsuper!(%3, Core.Function)
│        var"#25#26" = %3
│        Core._typebody!(%3, Core.svec())
└──      return nothing
)))
│   %2 = Core.TypeVar(:A)
│   %3 = var"#25#26"
│   %4 = Core.apply_type(Tuple, %2, B)
│   %5 = Core.apply_type(Type, %4)
│   %6 = Core.svec(%3, %5)
│   %7 = Core.svec(%2)
│   %8 = Core.svec(%6, %7, $(QuoteNode(:(#= REPL[16]:1 =#))))
│        $(Expr(:method, false, :(%8), CodeInfo(
    @ REPL[16]:1 within `none`
1 ─     return nothing
)))
│        #25 = %new(var"#25#26")
└──      return #25
))))