Inference-related internal method error during testing of StructWalk.jl

[maleadt]

Bisected to #50927; cc @vtjnash

As seen on PkgEval, during testing of StructWalk.jl:

Internal error: encountered unexpected error in runtime:
MethodError(f=Core.Compiler.:(⊑), args=(Core.Compiler.JLTypeLattice(), Vararg{Any}, Symbol), world=0x00000000000015e9)
jl_method_error_bare at /source/src/gf.c:2206
jl_method_error at /source/src/gf.c:2224
jl_lookup_generic_ at /source/src/gf.c:3055 [inlined]
ijl_apply_generic at /source/src/gf.c:3070
⊑ at ./compiler/typelattice.jl:530
⊑ at ./compiler/typelattice.jl:508
⊑ at ./compiler/typelattice.jl:432 [inlined]
⊑ at ./compiler/typelattice.jl:397
#348 at ./compiler/abstractlattice.jl:286 [inlined]
typevar_nothrow at ./compiler/tfuncs.jl:608 [inlined]
_builtin_nothrow at ./compiler/tfuncs.jl:2123
builtin_nothrow at ./compiler/tfuncs.jl:2463 [inlined]
builtin_effects at ./compiler/tfuncs.jl:2448

[Keno]

Something is leaking Vararg, but _builtin_nothrow does check, so it's something slightly more involved.

[maleadt]

MWE:

function getfields(obj::T) where T
    fnames = fieldnames(T)
    getfield.(Ref(obj), fnames)
end
function walk(f, WALKSTYLE, inner_walk, x)
    T, fields = typeof(x), getfields(x)
    map(inner_walk, fields)
    T(v...)
end
mapleaves(_, x) = walk(f, style, x -> mapleaves(fstyle, x), x)
mapleaves() = mapleaves(f, Ref)
mapleaves()

julia> mapleaves()
Internal error: encountered unexpected error in runtime:
MethodError(f=Core.Compiler.:(⊑), args=(Core.Compiler.JLTypeLattice(), Vararg{Any}, Symbol), world=0x00000000000016b2)
jl_method_error_bare at /home/tim/Julia/src/julia/src/gf.c:2215
jl_method_error at /home/tim/Julia/src/julia/src/gf.c:2233
jl_lookup_generic_ at /home/tim/Julia/src/julia/src/gf.c:3078 [inlined]
ijl_apply_generic at /home/tim/Julia/src/julia/src/gf.c:3093
⊑ at ./compiler/typelattice.jl:530
⊑ at ./compiler/typelattice.jl:508
⊑ at ./compiler/typelattice.jl:432 [inlined]
⊑ at ./compiler/typelattice.jl:397
#368 at ./compiler/abstractlattice.jl:288 [inlined]
typevar_nothrow at ./compiler/tfuncs.jl:613 [inlined]
_builtin_nothrow at ./compiler/tfuncs.jl:2180
builtin_nothrow at ./compiler/tfuncs.jl:2605 [inlined]
builtin_effects at ./compiler/tfuncs.jl:2523

[vtjnash]

Aka:

julia> code_typed((Any,)) do x; TypeVar(x...); end
ERROR: MethodError: no method matching ⊑(::Core.Compiler.JLTypeLattice, ::Core.TypeofVararg, ::Type{Symbol})