decltype as a return type in class member function

Question

I got error compiling below code.

struct B{
    double operator()(){
        return 1.0;
    }
};

struct A {
    auto func() -> decltype(b())
    {
             


        

Answer 1

Is it valid?

Your last example is well-formed, while the first one is not (so GCC is correct).

Paragraph 3.4.1/7 on unqualified name lookup specifies:

A name used in the definition of a class X outside of a member function body, default argument, brace-or- equal-initializer of a non-static data member, or nested class definition shall be declared in one of the following ways:

— before its use in class X or be a member of a base class of X (10.2), or

— [...]

And what follows are other conditions that do not apply in your case.