Nested struct breaks constexpr despite being identical to global ones

Question

I\'m having trouble with the following code:

template
constexpr int get(T vec) {
  return vec.get();
}

struct coord {
  constexpr int get(         


        

Answer 1

It is a constant expression.... eventually, as this shows you can see by moving i into main():

• http://ideone.com/lucfUi

The error messages are pretty clear what's going on, which is that foo::coord2::get() isn't defined yet, because member function definitions are delayed until the end of the enclosing class so that they can use members declared later.

It's a little surprising that the definition is delayed until the end of the outermost enclosing class, but you'd be even more surprised if foo::coord2::get() couldn't access foo::g.

The Standard agrees with the compiler, btw. Part of section 9.2p2 says

Within the class member-specification, the class is regarded as complete within function bodies, default arguments, exception-specifications, and brace-or-equal-initializers for non-static data members (including such things in nested classes).

Unfortunately, it's only inferred that the closing brace of the class declaration becomes the point-of-definition for these deferred regions. I believe it's a defect in the Standard that it doesn't say this explicitly.

See also:

• https://stackoverflow.com/a/11523155/103167