LLVM has an issue: llvm/llvm-project#43670

That issue marked as libc++ but actually every C++ standard library affected , and MSVC STL and libstdc++.

#include <compare>

auto b = 1 <=> 2 < 0;

int main() {}

clang-cl /std:c++20 -Wzero-as-null-pointer-constant main.cpp
main.cpp(3,20): warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]
auto b = 1 <=> 2 < 0;
                   ^
                   nullptr
1 warning generated.

I also created: https://reviews.llvm.org/D146372

Yes, after that patch we accept

auto b = 1 <=> 2 < (1-1);

but probably this is fine because the behavior is undefined. At least we don't warn on valid code.