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

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

[jeanmichael.celerier at gmail dot com]

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.

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

[johelegp at gmail dot com]

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

--- Comment #1 from Johel Ernesto Guerrero Peña <johelegp at gmail dot com> ---
This should block BUG101603.

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

[johelegp at gmail dot com]

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

--- Comment #2 from Johel Ernesto Guerrero Peña <johelegp at gmail dot com> ---
FWIW, it seems that you can use
<https://quuxplusone.github.io/blog/2022/01/04/test-constexpr-friendliness/>
to test the validity during constant evaluation.
It seems that an invalid constant expression implies a `false` result.
So I guard the check with a validity check:
<https://compiler-explorer.com/z/had59d6nv>.

```C++
#include <type_traits>
template<auto F> concept is_constant = requires { typename
std::integral_constant<int, (F(), 0)>; };
template<auto l, auto r>
constexpr bool implements() {
  using R = decltype(r);
  if constexpr (requires { static_cast<R>(l); })
#if !defined(__GNUC__) || defined(__clang__)
    return static_cast<R>(l) != r;
#else // Workaround <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71962>.
    return !is_constant<[]() { return static_cast<R>(l) != r; }> ||
static_cast<R>(l) != r;
#endif
  return false;
}
struct B { void f(); };
struct D0 : B { };
struct D1 : B { void f(); };
struct D2 : B { void f() const; };
// struct D3 : B { int f(); void f() const; };
static_assert(!implements<&D0::f, &B::f>());
static_assert(implements<&D1::f, &B::f>());
static_assert(!implements<&D2::f, &B::f>());
// static_assert(!implements<&D3::f, &B::f>());
```

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

[nathanieloshead at gmail dot com]

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

Nathaniel Shead <nathanieloshead at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |nathanieloshead at gmail dot com

--- Comment #3 from Nathaniel Shead <nathanieloshead at gmail dot com> ---
More generally this appears to be caused by '-fno-delete-null-pointer-checks'
causing constant folding not to occur. Minimised example:

  struct foo { void bar() { } };
  constexpr bool b = &foo::bar;

https://gcc.godbolt.org/z/x7csTzjfa

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

[johelegp at gmail dot com]

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

--- Comment #4 from Johel Ernesto Guerrero Peña <johelegp at gmail dot com> ---
They talk about `-fno-delete-null-pointer-checks` in BUG71962.