[Bug c++/125490] New: GCC accepts an invalid concept that compares sizeof(T) with nullptr

[qurong at ios dot ac.cn via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125490

            Bug ID: 125490
           Summary: GCC accepts an invalid concept that compares sizeof(T)
                    with nullptr
           Product: gcc
           Version: 15.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: qurong at ios dot ac.cn
  Target Milestone: ---

GCC 15.2.0 accepts the following C++20 program, while Clang 22 rejects it:

template<class T>
concept C1 = sizeof(T) == nullptr;

template<class T>
concept D1 = requires(T t) {
  { t } -> C1;
};

void f(D1 auto) {}

int main() {
  f(0);
}

GCC:gcc15 15.2.0
-std=c++20: accepts
-std=c++20 -pedantic-errors: accepts

Clang: clang++-22 22.0.0
-std=c++20: rejects
-std=c++20 -pedantic-errors: rejects

Clang diagnostic:
error: invalid operands to binary expression ('__size_t' (aka 'unsigned long')
and 'std::nullptr_t')