libstdc++: Fix std::counting_semaphore<> default max value

My recent (uncommitted) changes to support a 64-bit __platform_wait_t
for FreeBSD and Darwin revealed a problem in std::counting_semaphore.
When the default template argument is used and __platform_wait_t is a
64-bit type, the numeric_limits<__platform_wait_t>::max() value doesn't
fit in ptrdiff_t and so we get ptrdiff_t(-1), which fails a
static_assert in the class body.

The solution is to cap the value to PTRDIFF_MAX instead of allowing it
to go negative.

libstdc++-v3/ChangeLog:

	* include/bits/semaphore_base.h (__platform_semaphore::_S_max):
	Limit to PTRDIFF_MAX to avoid negative values.
	* testsuite/30_threads/semaphore/least_max_value.cc: New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

libstdc++-v3/include/bits/semaphore_base.h

@@ -173,8 +173,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   {
     using __count_type = __detail::__platform_wait_t;
 
-    static constexpr ptrdiff_t _S_max
-      = _Binary ? 1 : __gnu_cxx::__int_traits<__count_type>::__max;
+    static consteval ptrdiff_t
+    _S_calc_max()
+    {
+      if (_Binary)
+	return 1;
+      else if ((ptrdiff_t)__gnu_cxx::__int_traits<__count_type>::__max < 0)
+	return __gnu_cxx::__int_traits<ptrdiff_t>::__max;
+      else
+	return __gnu_cxx::__int_traits<__count_type>::__max;
+    }
+
+    static constexpr ptrdiff_t _S_max = _S_calc_max();
 
     constexpr explicit
     __platform_semaphore_impl(__count_type __count) noexcept

libstdc++-v3/testsuite/30_threads/semaphore/least_max_value.cc

@@ -0,0 +1,9 @@
+// { dg-do compile { target c++20 } }
+// { dg-require-effective-target gthreads { target { ! *-*-linux* } } }
+// { dg-require-effective-target hosted }
+
+#include <semaphore>
+
+std::counting_semaphore<> sem(0);
+std::counting_semaphore<> sem2(2);
+std::counting_semaphore sem3(3);