Clang-trunk rejects valid friend declaration of private base class with very confusing diagnostic

[wangbo15]

The following code is rejected by clang-trunk:

struct payload {};

struct base: private payload {
};

struct derived: base {
    friend class payload;
};

The diagnostic is hard to understand. It seems the compiler treats the private inheritance statement as a member declaration or something else here:

<source>:7:18: error: 'payload' is a private member of 'payload'
    7 |     friend class payload;
      |                  ^
<source>:3:14: note: constrained by private inheritance here
    3 | struct base: private payload {
      |              ^~~~~~~~~~~~~~~
<source>:1:8: note: member is declared here
    1 | struct payload {};
      |        ^
1 error generated.

GCC and MSVC both accept it.

Please see https://godbolt.org/z/4da5TbvYe

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Bo Wang (wangbo15)

The following code is rejected by clang-trunk:

struct payload {};

struct base: private payload {
};

struct derived: base {
    friend class payload;
};

The diagnostic is hard to understand. It seems the compiler treats the private inheritance statement as a member declaration or something else here:

<source>:7:18: error: 'payload' is a private member of 'payload'
    7 |     friend class payload;
      |                  ^
<source>:3:14: note: constrained by private inheritance here
    3 | struct base: private payload {
      |              ^~~~~~~~~~~~~~~
<source>:1:8: note: member is declared here
    1 | struct payload {};
      |        ^
1 error generated.

GCC and MSVC both accept it.

Please see https://godbolt.org/z/4da5TbvYe

[zygoloid]

Clang's behaviour appears correct to me. Unqualified name lookup is finding the injected class name of payload, which is a private member name of base due to the private inheritance, so is inaccessible from derived.

Maybe the diagnostic could be improved in a couple of ways: mentioning that this is the injected class name might help, suggesting rewriting as friend class ::payload; might be nice, and "'payload' is a private member of 'payload'" isn't accurate -- it's a public member but is private when accessed though base.