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

Question

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.

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.