[Bug c++/127272] New: Getting a warning about a code path guaranteed not to be taken

[eyalroz1 at gmx dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127272

            Bug ID: 127272
           Summary: Getting a warning about a code path guaranteed not to
                    be taken
           Product: gcc
           Version: 16.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eyalroz1 at gmx dot com
  Target Milestone: ---

(Following this StackOverflow Q&A: https://stackoverflow.com/q/80001781/1593077
and the code here: https://godbolt.org/z/dY4rj57aY)

Suppose I have a code within a (non-constexpr) if statement, which, if run, may
be problematic due to some feature of the class. Specifically, a `memmove()`
operation, which requires the class to have trivial copy-assignment.

Suppose also, that the condition can be decided at compile-time, negatively, so
that the compiler can know the condition is never met.

Should I be warned about the problematic execution in this case?

Here's a concrete example:

 #include <type_traits>
 #include <cstring>

  struct A {
      int x;
      A() {}
      A(int x_) : x(x_) {}
      A(A const& a) = default;
      A(A && a) = default;
      A& operator=(A const& other) { x = other.x; return *this; }
  };


  void foo() {
      A arr[2];
      A a { 123 };
      if (std::is_trivially_copy_assignable<A>::value) {
          std::memmove(arr, &a, sizeof(A));
      }
  }

this yields a compiler warning with GCC 16.2, but - not with clang 23.1; see
the GodBolt link above.

I believe GCC should also notice that the condition is necessarily false and
not  issue the warning.