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

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

[malekwryyy at gmail dot com]

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

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

[joseph at codesourcery dot com]

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

--- Comment #1 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
See the definitions of "named constant" and "compound literal constant".  
Array element accesses aren't allowed, and the example you have with "->" 
shouldn't be accepted either (although the standard rules for 
implementation-defined constant expressions probably allow implementations 
to accept such an example if they so choose).

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

[mpolacek at gcc dot gnu.org]

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

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mpolacek at gcc dot gnu.org
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Invalid then.

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

[malekwryyy at gmail dot com]

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

--- Comment #3 from Abdulmalek Almkainzi <malekwryyy at gmail dot com> ---
(In reply to joseph@codesourcery.com from comment #1)
> See the definitions of "named constant" and "compound literal constant".  
> Array element accesses aren't allowed, and the example you have with "->" 
> shouldn't be accepted either (although the standard rules for 
> implementation-defined constant expressions probably allow implementations 
> to accept such an example if they so choose).

Alright, I see how this is not a GCC bug and instead part of the standard. 

But may I ask how this is even the case? the expression is constant and can be
evaluated at compile time.

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

[joseph at codesourcery dot com]

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

--- Comment #4 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
The definition of constexpr in C2x is intentionally minimal, with 
potential for future expansion in subsequent standard revisions.  
Allowing array element accesses would run into needing an appropriate 
definition of exactly what cases are allowed and don't count as accessing 
the value of an object (presumably including requiring the array element 
index itself to be an integer constant expression within range for the 
array in question).