“Default member initializer needed within definition of enclosing class outside of member functions” - is my code ill-formed?

Question

#include 

struct foo
{
    int x{0};
    foo() noexcept = default;
    void f() noexcept(noexcept(std::declval())) {}
};

int main()
         


        

Answer 1

Your usage is fine.

• Your int x{0} clearly falls under the category non-static data member with initializer (C++11).
• The declval does not require a complete type--that's kind of the point of it, as explained in this nice exposition.

What's left? Compiler bug, I guess. How about this for evidence? Use a complete type z instead of foo in your declval.

#include 
struct z{};
struct foo
{
    int x{0};
    foo() noexcept = default;
    void f() noexcept( noexcept( std::declval() ) ) {}
};

int main()
{ 
}

Clang 4.0.0 on godbolt still errors the same way. Unfortunately I do not have clang 4.0.0 available on a machine to test, so I cannot not say whether it is Clang or godbolt for certain.