Program being compiled differently in 3 major C++ compilers. Which one is right?

Question

As an interesting follow-up (not of big practical importance though) to my previous question: Why does C++ allow us to surround the variable name in parentheses when declari

Answer 1

G++ is correct as it gives an error. Because the constructor could not be called directly in such a format without new operator. And although your code calls C::C, it looks like an constructor call. However, according to the C++11 standard 3.4.3.1, this is not a legal function call, or a type name (see Mike Seymour's answer).

Clang is wrong since it even does not call the correct function.

MSVC is something reasonable, but still it does not follow the standard.

Answer 2

GCC is correct, at least according to C++11 lookup rules. 3.4.3.1 [class.qual]/2 specifies that, if the nested name specifier is the same as the class name, it refers to the constructor not the injected class name. It gives examples:

B::A ba;           // object of type A
A::A a;            // error, A::A is not a type name
struct A::A a2;    // object of type A

It looks like MSVC misinterprets it as function-style cast expression creating a temporary C with y as a constructor parameter; and Clang misinterprets it as a declaration of a variable called y of type C.