“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 code is fine from what I can tell. Clang seems to struggle with the = default constructor rather than just defining a default constructor manually. It has the following spiel in its source code about it:

DR1351: If the brace-or-equal-initializer of a non-static data member invokes a defaulted default constructor of its class or of an enclosing class in a potentially evaluated subexpression, the program is ill-formed.

This resolution is unworkable: the exception specification of the default constructor can be needed in an unevaluated context, in particular, in the operand of a noexcept-expression, and we can be unable to compute an exception specification for an enclosed class.

Any attempt to resolve the exception specification of a defaulted default constructor before the initializer is lexically complete will ultimately come here at which point we can diagnose it.

I think it may be incorrectly picking up the error, personally. But it specifially mentions "defaulted default constructor".

The following seems to work:

#include 

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

int main()
{ 
}