[Bug c++/113852] New: -Wsign-compare doesn't warn on unsigned result types

["admin at computerquip dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113852

            Bug ID: 113852
           Summary: -Wsign-compare doesn't warn on unsigned result types
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: admin at computerquip dot com
  Target Milestone: ---

I haven't quite figured out the pattern here so the title may not be great.
Some code may help explain better: https://godbolt.org/z/d8cqd1WqP

```
#include <iostream>
#include <limits>
#include <cinttypes>

int main()
{
    uint16_t a1 = std::numeric_limits<unsigned short>::max();
    uint16_t a2 = std::numeric_limits<unsigned short>::max();

    /* a1 * a2 should be 4294836225 in math terms */
    uint64_t a3 = 4294836225;

    /*
     * The result of (a1 * a2) is of type int and the result is negative.
     * (a1 * a2) ends up as some bogus high number because the common
     * type here ends up as uint64_t and sign-extension occurs.
     */
    if ((a1 * a2) > a3) {
        std::cout << "this will print\n";
    }
}
```

Some observations I've noticed:
* If either a1 or a2 are constexpr, the warning will occur.
* This used to warn up until 8.1.
* Clang also doesn't warn here but MSVC will with /W3 or higher.
* This seems like a slight variation of
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101645