[Bug c/111421] New: constexpr not working with array subscript

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111421

            Bug ID: 111421
           Summary: constexpr not working with array subscript
           Product: gcc
           Version: 13.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: malekwryyy at gmail dot com
  Target Milestone: ---

Created attachment 55903
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55903&action=edit
the c program that results in the error

compiled with: -std=c2x -Wall -Wextra

When trying to subscript a constexpr array, the compiler says the expression is
not constant. 

I would expect if the index is an integer literal (or a constexpr integer) then
the resulting expression should still be constant.

I tried this with -std=c2x and -std=gnu2x

// program start

typedef struct S { int i; } S;

constexpr int a = (constexpr S){.i = 3}.i; // works as expected

constexpr int b = ( &(constexpr S){.i = 3} )->i; // works as expected

constexpr int y = (constexpr int[]){3}[0]; // doesn't work. Compiler says
expression not constant

int main()
{
    return 0;
}

// program end

the compiler error I get is:

<source>:7:19: error: initializer element is not constant
    7 | constexpr int y = (constexpr int[]){3}[0]; // doesn't work. Compiler
says expression not constant
      |                   ^
<source>:7:15: warning: 'y' defined but not used [-Wunused-const-variable=]
    7 | constexpr int y = (constexpr int[]){3}[0]; // doesn't work. Compiler
says expression not constant
      |               ^
<source>:5:15: warning: 'b' defined but not used [-Wunused-const-variable=]
    5 | constexpr int b = ( &(constexpr S){.i = 3} )->i; // works as expected
      |               ^
<source>:3:15: warning: 'a' defined but not used [-Wunused-const-variable=]
    3 | constexpr int a = (constexpr S){.i = 3}.i; // works as expected
      |               ^
Compiler returned: 1