Change the meaning of code by replacement of forward declaration with include

Question

Google C++ Style Guide states that

In extreme cases, replacing an #include with a forward declaration can silently change the meaning of code.

Can you please help me to find some examples?

Answer 1

Here are two cases. One of them is UB, the other I think is defined change of behavior (assuming no ODR or similar violations: ie, no call to foo ever sees the definition of A, but I am uncertain)

namespace N {
  struct B {};
  struct A;//:B{};
}

void foo(N::B*){
  std::cout << "B\n";
}
template<class T, class=std::enable_if_t<!std::is_convertible<T*,N::B*>{}>>
void foo(T*){
  std::cout << "T\n";
}

int main() {
  foo( (N::A*)0 );
}

replacing struct A; with struct A:B{}; will change which of the foo overloads are called.

In addition, delete A; will call ~A() if its is visible when delete A; is called. Otherwise, if there is a non-trivial destructor, we have UB. In this case, the meaning of the code changes in that it goes from UB to DB, which is I suppose a change of meaning.