For compatibility with MSVC, a friend declaration also act as a forward declaration...

For compatibility with MSVC, a friend declaration also act as a forward declaration if the tag name is not already declared. The tag name is declared in the next outermost non record scope.
Example:

class A {
  friend class B;
  B* b;
};
B* global_b;

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@132332 91177308-0d34-0410-b5e6-96231b3b80d8