https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100903
Bug ID: 100903
Summary: Bogus "zero as null pointer constant" warning
Product: gcc
Version: 11.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: terra at gnome dot org
Target Milestone: ---
Created attachment 50926
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50926&action=edit
Preprocesses source code
g++ emits a bogus warning when a std::partial_ordering value is compared
to a literal 0. See also bug 95242.
The standard is crazy-strict here. Anything other than a literal 0 is
undefined behaviour. Allowed: 0, 0LL, 0'0'0'0, 0x0. No good: +0, nullptr,
(void*)0, 0+0, '\0', (0,nullptr). gcc incorrectly allows nullptr and
(0,nullptr), but rejects the others.
The library tries to do that with a construct that mainly allows null pointer
constants. That leads to the warning when you actually supply the 0 you are
supposed to.
Suggestion: just use an int argument. That's wrong in different ways (and
might cause warnings with 0LL which is allowed), but it matches 0 better.
Doing this right is likely to require compiler support.
# cat uuu.C
#include <compare>
int
foo(std::partial_ordering po)
{
return po < 0;
}
# g++ -Wall -std=gnu++20 -Wzero-as-null-pointer-constant -c uuu.C
uuu.C: In function ‘int foo(std::partial_ordering)’:
uuu.C:6:15: warning: zero as null pointer constant
[-Wzero-as-null-pointer-constant]
6 | return po < 0;
| ^
# g++ -v
Using built-in specs.
COLLECT_GCC=/usr/local/products/gcc/11.1.0/bin/g++
COLLECT_LTO_WRAPPER=/usr/local/products/gcc/11.1.0/lib/gcc/x86_64-suse-linux/11.1.0/lto-wrapper
Target: x86_64-suse-linux
Configured with: ../../gcc-11.1.0/configure --enable-languages=c,c++,fortran
--enable-targets=x86_64-suse-linux,i686-suse-linux
--prefix=/usr/local/products/gcc/11.1.0 --with-gnu-as
--with-as=/usr/local/products/gcc/binutils-2.36.1/bin/as --with-gnu-ld
--with-ld=/usr/local/products/gcc/binutils-2.36.1/bin/ld --enable-link-mutex
--enable-gnu-indirect-functions --enable-linux-futex --enable-threads=posix
--enable-shared --enable-__cxa_atexit --enable-libstdcxx-allocator=new
x86_64-suse-linux
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 11.1.0 (GCC)
