Wzero-as-null-pointer-constant when using defaulted operator<=>

[mwinterb]

Bugzilla Link	50877
Version	trunk
OS	Windows NT
Attachments	possible patch for the problem
CC	@zygoloid

Extended Description

See bug #43670 for what is happening that produces the 0 -> nullptr conversion.

Compiling this code with -Wzero-as-null-pointer-constant

#include
struct Point {
int x{}, y{}, z{};
auto operator<=>(const Point&) const = default;
};

int main() {
return Point{} < Point{};
}

Produces three -Wzero-as-null-pointer-constant warnings, one for each member of Point. Fortunately suppressing the warning at the implementation site of operator<=> suppresses it for users.

Live example: https://gcc.godbolt.org/z/3n9Kf3baa

The fix is seemingly very close the fix to #​44325, but the build environment I have that can build clang doesn't have "enough" to actually run the tests so I don't know if my possible fix ends up suppressing the warning in other cases.

[ilya-biryukov]

Related issue in clang-tidy: #53961