Nested struct breaks constexpr despite being identical to global ones

Question

I'm having trouble with the following code:

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

struct coord {
  constexpr int get() const { return x; }
  int x;
};

struct foo {
    struct coord2 {
      constexpr int get() const { return x; }
      int x;
    };
    constexpr static coord f = { 5 };
    constexpr static int g = get(f); // works

    constexpr static coord2 h = { 5 };
    constexpr static int i = get(h); // doesn't work
};

constexpr coord foo::f;
constexpr foo::coord2 foo::h;

int main(){}

Essentially, get(f) is considered a constant expression, but get(h) is not. The only thing changed is that one uses a global struct coord, while the other uses a nested struct coord2. The structs' bodies are identical.

Why is this?

GCC error:

test.cpp:20:35: error: field initializer is not constant

Clang error:

test.cpp:20:26: error: constexpr variable 'i' must be initialized by a constant expression
    constexpr static int i = get(h); // doesn't work
                         ^   ~~~~~~
test.cpp:8:10: note: undefined function 'get' cannot be used in a constant expression
  return vec.get();
         ^
test.cpp:20:30: note: in call to 'get({5})'
    constexpr static int i = get(h); // doesn't work
                             ^
test.cpp:13:21: note: declared here
      constexpr int get() const { return x; }

score 17 · Accepted Answer · edited May 23 '17 at 12:19

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.

Nested struct breaks constexpr despite being identical to global ones

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