[Bug c++/107782] New: constexpr volatile variable is permitted to appear in a constant expression

[Bug c++/107782] New: constexpr volatile variable is permitted to appear in a constant expression

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107782

            Bug ID: 107782
           Summary: constexpr volatile variable is permitted to appear in
                    a constant expression
           Product: gcc
           Version: 12.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

This program

int main() {
    constexpr volatile int i = 0;
    constexpr volatile int j = i;
    return j;
}

is properly rejected by Clang since according to
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4093.html#1688
constexpr volatile variable cannot appear in a constant expression.

Unfortunately GCC (and MSVC) accept invalid program. Online demo:
https://gcc.godbolt.org/z/43ee65Peq

Related discussion: https://stackoverflow.com/q/74511594/7325599

[Bug c++/107782] a local constexpr volatile variable cannot be used to initialize a local constexpr volatile variable

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107782

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|https://gcc.gnu.org/bugzill |
                   |a/show_bug.cgi?id=87497,    |
                   |https://gcc.gnu.org/bugzill |
                   |a/show_bug.cgi?id=89553     |
             Status|UNCONFIRMED                 |NEW
            Summary|constexpr volatile variable |a local constexpr volatile
                   |is permitted to appear in a |variable cannot be used to
                   |constant expression         |initialize a local
                   |                            |constexpr volatile variable
   Last reconfirmed|                            |2022-11-21
     Ever confirmed|0                           |1

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed. 
Note GCC only does NOT rejects the local variable case.

That is take:
int main() {
  constexpr volatile int t = 1;
  static constexpr volatile int t1 = t;
  constexpr volatile int t2 = t;
  constexpr int t3 = t;
};

GCC will reject t1 and t3 correctly but not t2.
So GCC is mostly correct except for that one case while MSVC accepts all of the
above.