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.