[Bug c++/108736] New: pconcepts] multidimensional subscript operator reports invalid error in requires test

["ldalessandro at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108736

            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 = foo(1);

> <source>: In instantiation of 'decltype(auto) foo(auto:1, auto:2 ...) [with auto:1 = int; auto:2 = {}]':
> <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 empty.

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