spurious UndefVarError for static parameter matching

[nsajko]

Perplexing bug on master (but not on v1.10). EDIT: but see below, the basic bug existed already on v1.6

When I run this code in a fresh REPL session, all lines succeed:

f(::Type{<:Tuple{Vararg{A}}}) where {A} = A
f(Tuple{Vararg{Int}})
f(Tuple{Vararg{T}} where {T<:Int})

If the order of the calls to f is reversed, though, both calls error:

julia> f(::Type{<:Tuple{Vararg{A}}}) where {A} = A
f (generic function with 1 method)

julia> f(Tuple{Vararg{T}} where {T<:Int})
ERROR: UndefVarError: `A` not defined in static parameter matching
Suggestion: run Test.detect_unbound_args to detect method arguments that do not fully constrain a type parameter.
Stacktrace:
 [1] f(::Type{Tuple{Vararg{T}} where T<:Int64})
   @ Main ./REPL[1]:1
 [2] top-level scope
   @ REPL[2]:1

julia> f(Tuple{Vararg{Int}})
ERROR: UndefVarError: `A` not defined in static parameter matching
Suggestion: run Test.detect_unbound_args to detect method arguments that do not fully constrain a type parameter.
Stacktrace:
 [1] f(::Type{Tuple{Vararg{T}} where T<:Int64})
   @ Main ./REPL[1]:1
 [2] top-level scope
   @ REPL[3]:1

Version info:

Julia Version 1.11.0-DEV.1347
Commit 1f111e1326a (2024-01-20 19:02 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 8 × AMD Ryzen 3 5300U with Radeon Graphics
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, znver2)
Threads: 1 default, 0 interactive, 1 GC (on 8 virtual cores)

[nsajko]

This reproduces with b74daf5 (#51388), but not with its parent 29f2b2f. Even though b74daf5 only changes stdlibs.

[nsajko]

A similar reproducer, using a newly defined type (S) instead of Int, reproduces the bug all the way up to v1.6.7:

Works fine:

f(::Type{<:Tuple{Vararg{A}}}) where {A} = A
struct S end
f(Tuple{Vararg{S}})
f(Tuple{Vararg{T}} where {T<:S})

Errors:

f(::Type{<:Tuple{Vararg{A}}}) where {A} = A
struct S end
f(Tuple{Vararg{T}} where {T<:S})