friend declaration in derived class does not get honored

[llvmbot]

Bugzilla Link	35055
Version	trunk
OS	All
Reporter	LLVM Bugzilla Contributor
CC	@DougGregor

Extended Description

The following produces a ".... is a protected member of..." error while the friend declaration in the derived class is honored by gcc (multiple versions upto trunk), Visual Studio (multiple versions upto VS2017) and ICC.

Are they all wrongly accepting this code, or is this a Clang only issue ?

--8<--
class AFriend;

class Foo
{
protected:
static void Bar();
};

#if 1
class DerivedFoo
: public Foo
{
friend class AFriend;
};
#endif

class AFriend
{
public:
void CallFooBar()
{
Foo::Bar();
}
};
--8<--

[wheatman]

confirming that the situation is still the same in post 16 trunk(f05b58a)
https://godbolt.org/z/Gh7cjfMT5
With the code being rejected by clang and accepted by gcc and msvc

code

class AFriend;

class Foo {
	protected:
		static void Bar();
};

#if 1
class DerivedFoo :	public Foo {
	friend class AFriend;
};
#endif

class AFriend {
	public:
		void CallFooBar() {
			Foo::Bar();
		}
};

clang error

<source>:17:9: error: 'Bar' is a protected member of 'Foo'
   17 |                         Foo::Bar();
      |                              ^
<source>:5:15: note: declared protected here
    5 |                 static void Bar();
      |                             ^
1 error generated.
Compiler returned: 1

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[zygoloid]

This is CWG1873. Clang is correct: DerivedFoo befriending AFriend grants AFriend the right to access the protected members of DerivedFoo, including Bar, but only when named as members of DerivedFoo. Thus:

class AFriend {
public:
  void CallFooBar() {
    DerivedFoo::Bar(); // OK
    Foo::Bar(); // error
  }
};

I think the other compilers haven't implemented the CWG issue resolution yet.