c++ - 运算符转换、GCC 和 clang : which compiler is right?

Question

考虑以下代码:

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();
}

使用 GCC (4.9/5.1/6.1) 编译，但使用 clang (3.8/3.7) 编译失败。
返回的错误是:

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

在这种情况下哪个编译器是正确的，为什么？

注意

解决它是一个合格的问题 T:

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

问题不在于如何让它发挥作用。

Answer 1

我认为这是 clang 错误(以 #27807 提交)

来自 [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 ]

在 t = s.operator T(); 中，首先在 S 的类中查找 T，它应该会找到你的typedef 并因此最终调用 operator int()。