[Bug sanitizer/110936] New: if constexpr: member function pointers cannot be checked with ubsan

[jeanmichael.celerier at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110936

            Bug ID: 110936
           Summary: if constexpr: member function pointers cannot be
                    checked with ubsan
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: sanitizer
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jeanmichael.celerier at gmail dot com
                CC: dodji at gcc dot gnu.org, dvyukov at gcc dot gnu.org,
                    jakub at gcc dot gnu.org, kcc at gcc dot gnu.org, marxin at 
gcc dot gnu.org
  Target Milestone: ---

Repro:

    struct foo {
      void bar() { }
    };

    void process(auto f) {
      if constexpr(&decltype(f)::bar);
    }

    int main() {
      process(foo{});
    }


Works fine in normal compilation mode as expected, however with
-fsanitize=undefined it does not work: https://gcc.godbolt.org/z/9qvz89na8

    <source>:6:3: error: '(foo::bar != 0)' is not a constant expression
      if constexpr(&decltype(f)::bar);

I tested as far back as g++ 7.1 and it did not work either back then, and prior
versions did not have if constexpr.

It would be less of a problem if there was any way to do #if
defined(__SANITIZE_UNDEFINED__) or something similar but while asan has it,
ubsan does not seem to have any macro to enable detection of it.