c++ - Mixing class and struct

Question

I'm well aware of the difference between class and struct, however I'm struggling to authoritatively say if this is well defined:

// declare foo (struct)
struct foo;

// define foo (class)
class foo {
};

// instance of foo, claiming to be a struct again! Well defined?
struct foo bar;

// mixing class and struct like this upsets at least one compiler (names are mangled differently)
const foo& test() {
   return bar;
}

int main() {
   test();
   return 0;
}

If this is undefined behaviour can someone point me in the direction of an authoritative (i.e. chapter and verse from ISO) reference?

The compiler with problems handling this (Carbide 2.7) is relatively old and all the other compilers I've tried it on are perfectly happy with this, but clearly that doesn't prove anything.

My intuition was this ought to be undefined behaviour but I can't find anything to confirm this and I'm surprised that none of the GCC versions or Comeau so much as warned about it.

Answer 1

It looks to me like it's defined behavior. In particular, §9.1/2 says:

A declaration consisting solely of class-key identifier ; is either a redeclaration of the name in the current scope or a forward declaration of the identifier as a class name. It introduces the class name into the current scope.

The standard distinguishes between using class, struct or union when defining a class, but here, talking about a declaration, no such distinction is made -- using one class-key is equivalent to any other.