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

["mpolacek at gcc dot gnu.org" <gcc-bugzilla@gcc.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;
}