Clang-tidy: modernize-use-nullptr produces a false-positive on std:: strong_ordering (spaceship operator result)

[fastasturtle]

#include <compare>

int main() {
    auto res = 0 <=> 1;
    if (res < 0) {} // warning: use nullptr [modernize-use-nullptr]
}

https://gcc.godbolt.org/z/89W17v8j7

[llvmbot]

@llvm/issue-subscribers-clang-tidy

[PiotrZSL]

I'm not sure if this is false positive, simply if you put there any other number than 0, it wont compile, and in AST this 0 is implicit casted to pointer during construction of __cmp_cat::__unspec.

[fastasturtle]

I'm not sure if this is false positive, simply if you put there any other number than 0, it wont compile, and in AST this 0 is implicit casted to pointer during construction of __cmp_cat::__unspec.

@PiotrZSL while this is technically possible to use nullptr instead of 0, the comparison operator return value result is expected to be compared with 0 (similar to the return value of strcpy, etc) - see e.g. the original proposal.

The way it's worded in the standard is (if I'm reading it correctly) is here: https://eel.is/c++draft/cmp#categories.pre-3

[PiotrZSL]

https://reviews.llvm.org/D158928