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

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

[ldalessandro at gmail dot com]

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

[Bug c++/108736] [concepts] multidimensional subscript operator inside requires with variable template arguments is broken

[pinskia at gcc dot gnu.org]

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

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[concepts] multidimensional |[concepts] multidimensional
                   |subscript operator reports  |subscript operator inside
                   |invalid error in requires   |requires with variable
                   |test                        |template arguments is
                   |                            |broken
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-02-09

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed, even this is rejected:
```
    auto foo(auto a, auto... i) -> decltype(auto) {
        if constexpr (requires { a[i...]; }) {
            return a[i...];
        }
        else {
            return a;
        }
    }

    int x = foo(1, 1);
```

Note clang just ICEs.
Also the above works if it is not a variable argument auto ...

That is:
```
template<typename T, typename T1>
    auto foo(T a, T1 i) {
        if constexpr (requires { a[i]; }) {
            return a[i];
        }
        else {
            return a;
        }
    }

    int x = foo(1, 1);
```
Compiles just fine.

[Bug c++/108736] [concepts] multidimensional subscript operator inside requires with variable template arguments is broken

[ppalka at gcc dot gnu.org]

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

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |DUPLICATE
                 CC|                            |ppalka at gcc dot gnu.org

--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
dup of the recently fixed PR111493

*** This bug has been marked as a duplicate of bug 111493 ***