[Bug c++/97645] New: Rejects valid subscript expression on array of unknown bound in constant expression

[leni536 at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97645

            Bug ID: 97645
           Summary: Rejects valid subscript expression on array of unknown
                    bound in constant expression
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: leni536 at gmail dot com
  Target Milestone: ---

version: g++ (Compiler-Explorer-Build) 11.0.0 20201029 (experimental)

g++ rejects the following program in -std=c++20:

constexpr
int foo(const int (&arr)[]) {
    return arr[1];
}

constexpr
int bar(const int (&arr)[10]) {
    return foo(arr);
}

constinit auto x = bar({});

https://godbolt.org/z/Mq1GGf

This was possibly impossible prior to C++20 to subscript an array of unknown
bound in constexpr context. As of P0388 conversion from T[N] to T[] can happen
in constexpr context.