[Bug c++/114580] New: Bogus warning on if constexpr

[Bug c++/114580] New: Bogus warning on if constexpr

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114580

            Bug ID: 114580
           Summary: Bogus warning on if constexpr
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jakub at gcc dot gnu.org
  Target Milestone: ---

#include <type_traits>

template <typename T>
void foo ()
{
  if constexpr ((T) std::is_constant_evaluated ())
    ;
}

void
bar ()
{
  foo <bool> ();
}

emits bogus warning with -std=c++17 -Wall.
Once it (incorrectly) warns about
warning: ‘std::is_constant_evaluated’ always evaluates to false in a
non-‘constexpr’ function [-Wtautological-compare]
and once it correctly warns about
warning: ‘std::is_constant_evaluated’ always evaluates to true in ‘if
constexpr’ [-Wtautological-compare]
on the same line.  In reality, std::is_constant_evaluated () here always
evaluates to true, never to false.

[Bug c++/114580] Bogus warning on if constexpr

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114580

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2024-04-04
           Assignee|unassigned at gcc dot gnu.org      |jakub at gcc dot gnu.org
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |ASSIGNED

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 57873
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=57873&action=edit
gcc14-pr114580.patch

Untested fix.

[Bug c++/114580] Bogus warning on if constexpr

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114580

--- Comment #2 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:cfed80b9e4f562c99679739548df9369117dd791

commit r14-9861-gcfed80b9e4f562c99679739548df9369117dd791
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Apr 9 09:31:42 2024 +0200

    c++: Fix up maybe_warn_for_constant_evaluated calls [PR114580]

    When looking at maybe_warn_for_constant_evaluated for the trivial
    infinite loops patch, I've noticed that it can emit weird diagnostics
    for if constexpr in templates, first warn that std::is_constant_evaluted()
    always evaluates to false (because the function template is not constexpr)
    and then during instantiation warn that std::is_constant_evaluted()
    always evaluates to true (because it is used in if constexpr condition).
    Now, only the latter is actually true, even when the if constexpr
    is in a non-constexpr function, it will still always evaluate to true.

    So, the following patch fixes it to call maybe_warn_for_constant_evaluated
    always with IF_STMT_CONSTEXPR_P (if_stmt) as the second argument rather
than
    true if it is if constexpr with non-dependent condition etc.

    2024-04-09  Jakub Jelinek  <jakub@redhat.com>

            PR c++/114580
            * semantics.cc (finish_if_stmt_cond): Call
            maybe_warn_for_constant_evaluated with IF_STMT_CONSTEXPR_P
(if_stmt)
            as the second argument, rather than true/false depending on if
            it is if constexpr with non-dependent constant expression with
            bool type.

            * g++.dg/cpp2a/is-constant-evaluated15.C: New test.

[Bug c++/114580] Bogus warning on if constexpr

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114580

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

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

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Fixed.

[Bug c++/114580] Bogus warning on if constexpr

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114580

--- Comment #4 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-13 branch has been updated by Jakub Jelinek
<jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:ae3b6dea0445f9650cf1a684527efac06497f1b4

commit r13-8629-gae3b6dea0445f9650cf1a684527efac06497f1b4
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Apr 9 09:31:42 2024 +0200

    c++: Fix up maybe_warn_for_constant_evaluated calls [PR114580]

    When looking at maybe_warn_for_constant_evaluated for the trivial
    infinite loops patch, I've noticed that it can emit weird diagnostics
    for if constexpr in templates, first warn that std::is_constant_evaluted()
    always evaluates to false (because the function template is not constexpr)
    and then during instantiation warn that std::is_constant_evaluted()
    always evaluates to true (because it is used in if constexpr condition).
    Now, only the latter is actually true, even when the if constexpr
    is in a non-constexpr function, it will still always evaluate to true.

    So, the following patch fixes it to call maybe_warn_for_constant_evaluated
    always with IF_STMT_CONSTEXPR_P (if_stmt) as the second argument rather
than
    true if it is if constexpr with non-dependent condition etc.

    2024-04-09  Jakub Jelinek  <jakub@redhat.com>

            PR c++/114580
            * semantics.cc (finish_if_stmt_cond): Call
            maybe_warn_for_constant_evaluated with IF_STMT_CONSTEXPR_P
(if_stmt)
            as the second argument, rather than true/false depending on if
            it is if constexpr with non-dependent constant expression with
            bool type.

            * g++.dg/cpp2a/is-constant-evaluated15.C: New test.

    (cherry picked from commit cfed80b9e4f562c99679739548df9369117dd791)

[Bug c++/114580] Bogus warning on if constexpr

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114580

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Fixed for 13.3 too.