effect: mark Core.TypeofVararg as ismutationfree (#52586)

Found in https://buildkite.com/julialang/julia-master/builds/31309#018c7f0b-aada-4017-bb2f-44001593ac6b. On master keyword call might taint `inaccessiblememonly`. MWE: ```julia julia> foo(; kws...) = 1 foo (generic function with 1 method) julia> Base.infer_effects(foo, Tuple{}) (+c,+e,+n,+t,+s,!m,+u) ``` which is caused by ```julia julia> Base.infer_effects(()->Vararg) (+c,+e,+n,+t,+s,!m,+u) ``` Co-authored-by: Shuhei Kadowaki <40514306+aviatesk@users.noreply.github.com>
JuliaLang · Dec 21, 2023 · 631e7ef · 631e7ef
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diff --git a/src/jltypes.c b/src/jltypes.c
@@ -2785,6 +2785,7 @@ void jl_init_types(void) JL_GC_DISABLED
     XX(vararg);
     // It seems like we probably usually end up needing the box for kinds (often used in an Any context), so force it to exist
     jl_vararg_type->name->mayinlinealloc = 0;
+    jl_vararg_type->ismutationfree = 1;
 
     jl_svec_t *anytuple_params = jl_svec(1, jl_wrap_vararg((jl_value_t*)jl_any_type, (jl_value_t*)NULL, 0));
     jl_anytuple_type = jl_new_datatype(jl_symbol("Tuple"), core, jl_any_type, anytuple_params,

diff --git a/test/compiler/effects.jl b/test/compiler/effects.jl
@@ -1357,6 +1357,9 @@ end
 @test set_a52531!(1) == 1
 @test get_a52531() == 1
 
+@test Core.Compiler.is_inaccessiblememonly(Base.infer_effects(identity∘identity, Tuple{Any}))
+@test Core.Compiler.is_inaccessiblememonly(Base.infer_effects(()->Vararg, Tuple{}))
+
 # pointerref nothrow for invalid pointer
 @test !Core.Compiler.intrinsic_nothrow(Core.Intrinsics.pointerref, Any[Type{Ptr{Vector{Int64}}}, Int, Int])
 @test !Core.Compiler.intrinsic_nothrow(Core.Intrinsics.pointerref, Any[Type{Ptr{T}} where T, Int, Int])