[Bug c++/96278] New: three-way comparison operator comparison to 0: spurious nullptr warning

[gumby@henkel-wallace.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96278

            Bug ID: 96278
           Summary: three-way comparison operator comparison to 0:
                    spurious nullptr warning
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gumby@henkel-wallace.org
  Target Milestone: ---

Bug in g++ 10.1.0 with libstdc++

compile following program doing `g++-10 -std=c++20
-Wzero-as-null-pointer-constant`

#include <iostream>
#include <compare>

int main () {
  bool result { (1 <=> 2) < 0 };
  std::cout << (result ? "Yes" : "No") << std::endl;
}

produces the error:

   foo.cc:5:27: warning: zero as null pointer constant
[-Wzero-as-null-pointer-constant]

Also compiler-synthesized comparisons will generate this which is especially
confusing as they can happen when calling a library function that needs the '<'
operator.

the exact version of GCC: 10.1.0

the system type: MacOS 10.15 (and Ubuntu 18.4 LTS using apt-get of gcc 10)

the options given when GCC was configured/built: (homebrew version): Configured
with: ../configure --build=x86_64-apple-darwin19
--prefix=/usr/local/Cellar/gcc/10.1.0
--libdir=/usr/local/Cellar/gcc/10.1.0/lib/gcc/10 --disable-nls
--enable-checking=release --enable-languages=c,c++,objc,obj-c++,fortran
--program-suffix=-10 --with-gmp=/usr/local/opt/gmp
--with-mpfr=/usr/local/opt/mpfr --with-mpc=/usr/local/opt/libmpc
--with-isl=/usr/local/opt/isl --with-system-zlib --with-pkgversion='Homebrew
GCC 10.1.0' --with-bugurl=https://github.com/Homebrew/homebrew-core/issues
--disable-multilib --with-native-system-header-dir=/usr/include
--with-sysroot=/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk
SED=/usr/bin/sed