运营商演员，GCC和clang：哪个编译器是对的？

Question

Consider the following code:

struct S {
    using T = int;
    operator T() { return 42; }
};

int main() {
    S s;
    S::T t = s;
    // Is the following line correct?
    t = s.operator T();
}

It compiles with GCC (4.9/5.1/6.1), but it fails to compile with clang (3.8/3.7).
The error returned is:

error: unknown type name 'T'; did you mean 'S::T'?

Which compiler is right in this case and why?

Note

Solving it is a matter of qualifying T :

t = s.operator S::T();

The question is not about how to make it work.

Answer 1

I believe this is clang bug (submitted as #27807 )

From [basic.lookup.classref] :

If the id-expression is a conversion-function-id , its conversion-type-id is first looked up in the class of the object expression and the name, if found, is used. Otherwise it is looked up in the context of the entire postfix-expression . In each of these lookups, only names that denote types or templates whose specializations are types are considered. [ Example:

 struct A { }; namespace N { struct A { void g() { } template <class T> operator T(); }; } int main() { N::A a; a.operator A(); // calls N::A::operator N::A } 

—end example ]

In t = s.operator T(); , T is first looked up in the class of S , which should find your typedef and hence end up calling operator int() .