[Bug c++/100995] New: Extend std::is_constant_evaluated in if warning

[Bug c++/100995] New: Extend std::is_constant_evaluated in if warning

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100995

            Bug ID: 100995
           Summary: Extend std::is_constant_evaluated in if warning
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mpolacek at gcc dot gnu.org
  Target Milestone: ---

Let's extend the existing warning to detect more cases, as in
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1938r3.html#compiler-warnings

constexpr int f1() {
  if constexpr (!std::is_constant_evaluated() && sizeof(int) == 4) { //
warning: always true
    return 0;
  }
  if (std::is_constant_evaluated()) {
    return 42;
  } else {
    if constexpr (std::is_constant_evaluated()) { // warning: always true
      return 0;
    }
  }
  return 7;
}


consteval int f2() {
  if (std::is_constant_evaluated() && f1()) { // warning: always true
    return 42;
  }
  return 7;
}


int f3() {
  if (std::is_constant_evaluated() && f1()) { // warning: always false
    return 42;
  }
  return 7;
}

[Bug c++/100995] Extend std::is_constant_evaluated in if warning

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100995

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
           Keywords|                            |diagnostic
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2021-06-09

[Bug c++/100995] Extend std::is_constant_evaluated in if warning

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100995

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
namespace std {
  constexpr inline bool
  is_constant_evaluated () noexcept
  {
    return __builtin_is_constant_evaluated ();
  }
}

int
foo ()
{
  if (std::is_constant_evaluated ())
    return 1;
  return 0;
}

constexpr int
bar ()
{
  if (std::is_constant_evaluated ())
    return 1;
  if constexpr (std::is_constant_evaluated ())
    return 2;
  return 0;
}

consteval int
baz ()
{
  if (std::is_constant_evaluated ())
    return 1;
  return 0;
}

In foo, we know it will always evaluate to false, in baz always to true.
In non-constexpr functions it will not evaluate everywhere to false, so one
would need to leave out e.g. initializers of static variables and the other
spots which are manifestly constant evaluated.
But say if conditions except for statement expressions in them and if it is not
the init might be always ok and good enough for the warning.

[Bug c++/100995] Extend std::is_constant_evaluated in if warning

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100995

--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:26dbe85a3781af913639b17bc966f4a0b8209f3b

commit r12-1375-g26dbe85a3781af913639b17bc966f4a0b8209f3b
Author: Marek Polacek <polacek@redhat.com>
Date:   Wed Jun 9 15:18:39 2021 -0400

    c++: Extend std::is_constant_evaluated in if warning [PR100995]

    Jakub pointed me at
   
<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1938r3.html#compiler-warnings>
    which shows that our existing warning could be extended to handle more
    cases.  This patch implements that.

    A minor annoyance was handling macros, in libstdc++ we have

      reference operator[](size_type __pos) {
          __glibcxx_assert(__pos <= size());
          ...
      }

    wherein __glibcxx_assert expands to

      if (__builtin_is_constant_evaluated() && !bool(__pos <= size())
        ...

    but I'm of a mind to not warn on that.

    Once consteval if makes it in, we should tweak this warning one more
    time.

            PR c++/100995

    gcc/cp/ChangeLog:

            * constexpr.c (maybe_constexpr_fn): New.
            * cp-tree.h (maybe_constexpr_fn): Declare.
            * semantics.c (find_std_constant_evaluated_r): New.
            (maybe_warn_for_constant_evaluated): New.
            (finish_if_stmt_cond): Call it.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/is-constant-evaluated9.C: Add dg-warning.
            * g++.dg/cpp2a/is-constant-evaluated12.C: New test.

[Bug c++/100995] Extend std::is_constant_evaluated in if warning

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100995

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

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

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Done.