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From: "ldalessandro at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/108736] New: pconcepts] multidimensional subscript operator
 reports invalid error in requires test
Date: Thu, 09 Feb 2023 07:25:59 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 13.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108736

            Bug ID: 108736
           Summary: pconcepts] multidimensional subscript operator reports
                    invalid error in requires test
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ldalessandro at gmail dot com
  Target Milestone: ---

The following code produces an error when evaluating the concept.

    auto foo(auto a, auto... i) -> decltype(auto) {
        if constexpr (requires { a[i...]; }) {
            return a[i...];
        }
        else {
            return a;
        }
    }

    int x =3D foo(1);

> <source>: In instantiation of 'decltype(auto) foo(auto:1, auto:2 ...) [wi=
th auto:1 =3D int; auto:2 =3D {}]':
> <source>:10:12:   required from here
> <source>:2:31: error: built-in subscript operator without expression list
>     2 |     if constexpr (requires { a[i...]; }) {
>       |                              ~^
> Compiler returned: 1

Workaround: specialize on sizeof...(i) to avoid this test when i... is empt=
y.

Live example: https://godbolt.org/z/K3oezd1qz=