[Bug c++/120449] New: g++ -Wtype-limits without -P doesn't warn on unsigned compare to zero

[zmajeed at sbcglobal dot net via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120449

            Bug ID: 120449
           Summary: g++ -Wtype-limits without -P doesn't warn on unsigned
                    compare to zero
           Product: gcc
           Version: 15.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zmajeed at sbcglobal dot net
  Target Milestone: ---

The is from libstdc++-v3/include/bits/semaphore_base.h

struct __atomic_semaphore {
  explicit __atomic_semaphore(__detail::__platform_wait_t __count) noexcept:
_counter(__count)     {
    __glibcxx_assert(__count >= 0 && __count <= _S_max);
  }
};

On Windows Cygwin, __platform_wait_t is unsigned long from
libstdc++-v3/include/bits/atomic_wait.h

#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
#define _GLIBCXX_HAVE_PLATFORM_WAIT 1
    using __platform_wait_t = int;
    inline constexpr size_t __platform_wait_alignment = 4;
#else
# if ATOMIC_LONG_LOCK_FREE == 2
    using __platform_wait_t = unsigned long;
# else
    using __platform_wait_t = unsigned int;
# endif
    inline constexpr size_t __platform_wait_alignment
      = __alignof__(__platform_wait_t);
#endif

Here's the single-line typelimits.cpp testfile

#include <semaphore>

g++ -c --save-temps -Wtype-limits -std=c++20 -P typelimits.cpp

typelimits.ii: In constructor
‘std::__atomic_semaphore::__atomic_semaphore(std::__detail::__platform_wait_t)’:
typelimits.ii:10140:47: warning: comparison of unsigned expression in ‘>= 0’ is
always true [-Wtype-limits]
10140 |       do { if (__builtin_expect(!bool(__count >= 0 && __count <=
_S_max), false))
std::__glibcxx_assert_fail("/usr/lib/gcc/x86_64-pc-cygwin/15/include/c++/bits/semaphore_base.h",
178, __PRETTY_FUNCTION__, "__count >= 0 && __count <= _S_max"); } while
(false);
      |                                       ~~~~~~~~^~~~

But there's no warning from -Wtype-limits when -P is removed

g++ -c --save-temps -Wtype-limits -std=c++20 typelimits.cpp

grep -En '__count >= 0|using __platform_wait_t' typelimits.ii

6845:    using __platform_wait_t = unsigned long;
14948:      do { if (__builtin_expect(!bool(__count >= 0 && __count <= _S_max),
false))
std::__glibcxx_assert_fail("/usr/lib/gcc/x86_64-pc-cygwin/15/include/c++/bits/semaphore_base.h",
178, __PRETTY_FUNCTION__, "__count >= 0 && __count <= _S_max"); } while
(false);

On Linux __platform_wait_t is int so -Wtype-limits doesn't apply to the line