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From: "danakj at orodu dot net" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/108067] New: Miscompilation with friend function with
 parameter pack: mismatched argument pack lengths
Date: Mon, 12 Dec 2022 06:11:30 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 12.2.1
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: danakj at orodu dot net
X-Bugzilla-Status: UNCONFIRMED
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108067

            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 =3D 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=