[Bug c++/91483] Poor diagnostic on trying to take constexpr reference to non-static object

[Bug c++/91483] Poor diagnostic on trying to take constexpr reference to non-static object

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91483

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Related test:

void
foo ()
{
  constexpr int a = 0;
  constexpr const int *p = &a;
}

We just say
error: ‘& a’ is not a constant expression
but that's inadequate.  clang++ now says

note: address of non-static constexpr variable 'a' may differ on each
invocation of the enclosing function; add 'static' to give it a constant
address

[Bug c++/91483] Poor diagnostic on trying to take constexpr reference to non-static object

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91483

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
The error comes from verify_constant, which doesn't explain anything. 
verify_constant uses reduced_constant_expression_p which just says yes/no but
doesn't explain anything.  reduced_constant_expression_p uses the middle-end
initializer_constant_valid_p but that's not going to say anything, either.

Either we need a version of reduced_constant_expression_p that actually says
what's wrong, or add a function that, when given an expression that isn't
reduced_constant_expression_p, will look for known problematical cases, like
the one above.

[Bug c++/91483] Poor diagnostic on trying to take constexpr reference to non-static object

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91483

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:b78cedc6b5bc062717a3e0efb10da8e19af1c422

commit r14-3718-gb78cedc6b5bc062717a3e0efb10da8e19af1c422
Author: Marek Polacek <polacek@redhat.com>
Date:   Fri Sep 1 17:26:01 2023 -0400

    c++: improve verify_constant diagnostic [PR91483]

    When verify_constant complains, it's pretty terse.  Consider

      void test ()
      {
        constexpr int i = 42;
        constexpr const int *p = &i;
      }

    where it says "'& i' is not a constant expression".  OK, but why?

    With this patch, we say:

    b.C:5:28: error: '& i' is not a constant expression
        5 |   constexpr const int *p = &i;
          |                            ^~
    b.C:5:28: note: pointer to 'i' is not a constant expression
    b.C:4:17: note: address of non-static constexpr variable 'i' may differ on
each invocation of the enclosing function; add 'static' to give it a constant
address
        4 |   constexpr int i = 42;
          |                 ^
          |                 static

    which brings g++ on par with clang++.

            PR c++/91483

    gcc/cp/ChangeLog:

            * constexpr.cc (verify_constant_explain_r): New.
            (verify_constant): Call it.

    gcc/testsuite/ChangeLog:

            * g++.dg/diagnostic/constexpr3.C: New test.

[Bug c++/91483] Poor diagnostic on trying to take constexpr reference to non-static object

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91483

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED

--- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Should be fixed now.