[Bug c++/108067] New: Miscompilation with friend function with parameter pack: mismatched argument pack lengths

[Bug c++/108067] New: Miscompilation with friend function with parameter pack: mismatched argument pack lengths

[danakj at orodu dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108067

            Bug ID: 108067
           Summary: Miscompilation with friend function with parameter
                    pack: mismatched argument pack lengths
           Product: gcc
           Version: 12.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: danakj at orodu dot net
  Target Milestone: ---

The following code compiles in Clang and MSVC but not in GCC.

Repros in GCC 10, 11, 12, and trunk.

```
template <class A, class B>
static constexpr bool C = true;

template <class... Ts>
struct Foo {
    template <class... Us>
        requires((... && C<Ts, Us>))
    void bar(const Foo<Us...>& r) {}

    template <class... Us>
        requires((... && C<Ts, Us>))
    friend void baz(const Foo& l, const Foo<Us...>& r) {}
};

int main() {
    Foo<int, int> x;

    x.bar(Foo<int, int>());  // Works.

    baz(x, Foo<int, int>());  // Doesn't work.
}
```

godbolt: https://godbolt.org/z/3Ybofbcoc

<source>: In instantiation of 'struct Foo<int, int>':
<source>:16:19:   required from here
<source>:12:17: error: mismatched argument pack lengths while expanding 'C<Ts,
Us>'
   12 |     friend void baz(const Foo& l, const Foo<Us...>& r) {}
      |                 ^~~
ASM generation compiler returned: 1
<source>: In instantiation of 'struct Foo<int, int>':
<source>:16:19:   required from here
<source>:12:17: error: mismatched argument pack lengths while expanding 'C<Ts,
Us>'
   12 |     friend void baz(const Foo& l, const Foo<Us...>& r) {}
      |                 ^~~
Execution build compiler returned: 1

[Bug c++/108067] Miscompilation with friend function with parameter pack: mismatched argument pack lengths

[danakj at orodu dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108067

--- Comment #1 from danakj at orodu dot net ---
Notably, this occurs with friend operator== or operator<=>.

[Bug c++/108067] Miscompilation with friend function with parameter pack: mismatched argument pack lengths

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108067

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2022-12-12
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
                 CC|                            |jason at gcc dot gnu.org,
                   |                            |marxin at gcc dot gnu.org

--- Comment #2 from Martin Liška <marxin at gcc dot gnu.org> ---
Confirmed.

[Bug c++/108067] Miscompilation with friend function with parameter pack: mismatched argument pack lengths

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108067

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot gnu.org
                 CC|                            |ppalka at gcc dot gnu.org
             Status|NEW                         |ASSIGNED

[Bug c++/108067] Miscompilation with friend function with parameter pack: mismatched argument pack lengths

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108067

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
             Status|ASSIGNED                    |RESOLVED

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
dup

*** This bug has been marked as a duplicate of bug 107853 ***