[Bug c++/115343] New: Member name lookup does not consider equivalent type aliases from different base classes to be equivalent.

[Bug c++/115343] New: Member name lookup does not consider equivalent type aliases from different base classes to be equivalent.

[email at miropalmu dot cc]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115343

            Bug ID: 115343
           Summary: Member name lookup does not consider equivalent type
                    aliases from different base classes to be equivalent.
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: email at miropalmu dot cc
  Target Milestone: ---

In following code foo::tag and bar::tag are type aliases to the same type so
member name lookup in foobar::tag should merge them and not be ambiguous.

However GCC 15.0.0 20240604 gives error about ambiguity.


```https://godbolt.org/z/fsK7ET4bY

struct tag { };

struct foo {
    using tag = tag;
};

struct bar { 
    using tag = tag;
};

struct foobar : foo, bar { };

int main() {
    foobar::tag _;
}
```

Clang is able to compile the code and it would make sense to no be ambiguous
but I'm not 100% sure, so someone with more familiarity with
[class.member.lookup] section of the standard can correct me if I'm wrong.

[Bug c++/115343] Member name lookup does not consider equivalent type aliases from different base classes to be equivalent.

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115343

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note the original example is invalid code to begin.
```
<source>:5:11: warning: declaration of 'using foo::tag = struct tag' changes
meaning of 'tag' [-Wchanges-meaning]
    5 |     using tag = tag;
      |           ^~~
<source>:5:17: note: used here to mean 'struct tag'
    5 |     using tag = tag;
      |                 ^~~
<source>:2:8: note: declared here
    2 | struct tag { };
      |        ^~~
```


But a simple change to the source:
```
struct tag { };

struct foo {
    using tag1 = tag;
};

struct bar { 
    using tag1 = tag;
};

struct foobar : foo, bar { };

int main() {
    foobar::tag1 _;
}
```

Gets the questionable ambiguous error message without a warning.

[Bug c++/115343] Member name lookup does not consider equivalent type aliases from different base classes to be equivalent.

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115343

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
MSVC rejects the code:

<source>(15): error C2385: ambiguous access of 'tag1'
<source>(15): note: could be the 'tag1' in base 'foo'
<source>(15): note: or could be the 'tag1' in base 'bar'

So does EDG:

"<source>", line 15: error: "foobar::tag1" is ambiguous
      foobar::tag1 _;
              ^

1 error detected in the compilation of "<source>".


Which makes this me 99% sure this is a clang issue.