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From: "jlaros at fixedpoint dot nl" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/107623] New: Using a parameter pack twice results in
 ambiguous overloaded function.
Date: Thu, 10 Nov 2022 20:15:47 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 12.2.0
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: jlaros at fixedpoint dot nl
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=3D107623

            Bug ID: 107623
           Summary: Using a parameter pack twice results in ambiguous
                    overloaded function.
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jlaros at fixedpoint dot nl
  Target Milestone: ---

The following example does not compile:

    template <class R, class... Args>
    void test(R (*)(Args...), Args...) {}

    template <class... Args>
    void test(void (*)(Args...), Args...) {}

    int main() {
      void (*f)(int) {};
      test(f, 1);
    }

Example:

    $ g++ -Wall -Wextra -pedantic x.cc
    x.cc: In function =E2=80=98int main()=E2=80=99:
    x.cc:11:7: error: call of overloaded =E2=80=98test(void (*&)(int), int)=
=E2=80=99 is
ambiguous
       11 |   test(f, 1);
          |   ~~~~^~~~~~
    x.cc:2:6: note: candidate: =E2=80=98void test(R (*)(Args ...), Args ...=
) [with R =3D
void; Args =3D {int}]=E2=80=99
        2 | void test(R (*)(Args...), Args...) {}
          |      ^~~~
    x.cc:5:6: note: candidate: =E2=80=98void test(void (*)(Args ...), Args =
...) [with
Args =3D {int}]=E2=80=99
        5 | void test(void (*)(Args...), Args...) {}
          |      ^~~~

This might be a bug because the following variants on the same theme do com=
pile
without errors or warnings:

    template <class R, class T>
    void test(R (*)(T), T) {}

    template <class T>
    void test(void (*)(T), T) {}

And:

    template <class R, class... FArgs, class... Args>
    void test(R (*)(FArgs...), Args...) {}

    template <class... FArgs, class... Args>
    void test(void (*)(FArgs...), Args...) {}

The first example does compile when using clang (version 14.0.6).=