inference: fix istuple check in apply_type_tfunc (#52585)

close #51927

(cherry picked from commit 58fac69)

Author: N5N3

base/compiler/tfuncs.jl

@@ -1731,7 +1731,7 @@ const _tvarnames = Symbol[:_A, :_B, :_C, :_D, :_E, :_F, :_G, :_H, :_I, :_J, :_K,
         end
         return allconst ? Const(ty) : Type{ty}
     end
-    istuple = isa(headtype, Type) && (headtype == Tuple)
+    istuple = headtype === Tuple
     if !istuple && !isa(headtype, UnionAll) && !isvarargtype(headtype)
         return Union{}
     end

test/compiler/inference.jl

@@ -5129,3 +5129,8 @@ let TV = TypeVar(:T)
     some = Some{Any}((TV, t))
     @test abstract_call_unionall_vararg(some) isa UnionAll
 end
+
+# Issue #51927
+let 𝕃 = Core.Compiler.fallback_lattice
+    @test apply_type_tfunc(𝕃, Const(Tuple{Vararg{Any,N}} where N), Int) == Type{NTuple{_A, Any}} where _A
+end