friend definition makes function visible when it should not

[ryanhaining]

Bugzilla Link	40233
Version	trunk
OS	Linux
CC	@DougGregor,@zygoloid

Extended Description

Based on this stackoverflow question: https://stackoverflow.com/q/54055638/1013719

gcc correctly rejects the following, while clang accepts:

class Cls {
    friend void func(int) { }
    friend void ::func(int);
};

with:

error: ‘void func(int)’ has not been declared within ‘::’
     friend void ::func(int);
                 ^~
note: only here as a ‘friend’
     friend void func(int) { }
                 ^~~~

The friend definition should not make func visible by http://eel.is/c++draft/namespace.memdef#3.sentence-2

The friend declaration does not by itself make the name visible to unqualified lookup or qualified lookup.

[zygoloid]

Under wg21.link/cwg1477, this is intended to be valid: the friend declaration doesn't make the function visible to qualified or unqualified name lookup, but does make it visible to redeclaration lookup.

However, see also wg21.link/cwg1900, which re-raises this question. Current CWG discussion on that suggests that we're probably going to make examples such as this ill-formed.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (29c14d05-4b2c-4b95-ba37-19ba3af0de5b)

| | | | --- | --- | | Bugzilla Link | [40233](https://llvm.org/bz40233) | | Version | trunk | | OS | Linux | | CC | @DougGregor,@zygoloid |

Extended Description

Based on this stackoverflow question: https://stackoverflow.com/q/54055638/1013719

gcc correctly rejects the following, while clang accepts:

class Cls {
    friend void func(int) { }
    friend void ::func(int);
};

with:

error: ‘void func(int)’ has not been declared within ‘::’
     friend void ::func(int);
                 ^~
note: only here as a ‘friend’
     friend void func(int) { }
                 ^~~~

The friend definition should not make func visible by http://eel.is/c++draft/namespace.memdef#3.sentence-2

> The friend declaration does not by itself make the name visible to unqualified lookup or qualified lookup.

[shafik]

CWG1900 was resolved by p1787r6 and it looks like basic.lookup.unqual p6 says this is ok.

All implementations now accept it: https://godbolt.org/z/s7vnjYjbf

Although gcc does w/ a diagnostic that :: is ignored and it does not make it globally visible. This feels like a weird result.

[Endilll]

In post-P1787R6 wording, this example is well-formed. I can suggest my write-up about CWG1900 in description of #106117 that explains what's going on in this space. Below I'll do targeted explanations for this issue.

GCC used to behave like it needs to do a regular lookup for the second (qualified) declaration, and if you do that, then yeah, declarations of friend functions do not bind names, so nothing to be found by name lookup. But as Richard pointed out, terminal names of declarator-ids are not subjects to regular name lookup.

Under wg21.link/cwg1477, this is intended to be valid: the friend declaration doesn't make the function visible to qualified or unqualified name lookup, but does make it visible to redeclaration lookup.

It is still true. In the current wording, "redeclaration lookup" is "nominable" and all business around it.

However, see also wg21.link/cwg1900, which re-raises this question. Current CWG discussion on that suggests that we're probably going to make examples such as this ill-formed.

I believe in the end such examples were deemed well-formed.

it looks like basic.lookup.unqual p6 says this is ok.

Those adjustments to name lookup do not help when there are no names to be found, because no names were bound by friend declarations.

[shafik]

@Endilll do we have a test that covers this case? If not we should add one.

[Endilll]

@Endilll do we have a test that covers this case? If not we should add one.

Excellent suggestion. While I was working on it, I re-read relevant pieces of wording and my old findings documented all over the place, and I realized that https://eel.is/c++draft/dcl.meaning#general-2.2.2 requires friend declarations that use qualified names to successfully find their corresponding declarations via name lookup. In other words, it's not enough that the first friend declaration is nominable in global namespace.