[Bug libstdc++/112882] [14 Regression] std::clamp no longer usable in header only mode

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112882

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
That change was broken anyway: when _GLIBCXX_ASSERTIONS was not defined, the
condition in the assertion is if constexpr (is_constant_evaluated()) which is
always true, even when not actually doing constant eval.

I'll test this:

--- a/libstdc++-v3/include/bits/c++config
+++ b/libstdc++-v3/include/bits/c++config
@@ -538,6 +538,7 @@ namespace std
   // This can be used without checking if the compiler supports the feature.
   // The macro _GLIBCXX_HAVE_IS_CONSTANT_EVALUATED can be used to check if
   // the compiler support is present to make this function work as expected.
+  __attribute__((__always_inline__))
   _GLIBCXX_CONSTEXPR inline bool
   __is_constant_evaluated() _GLIBCXX_NOEXCEPT
   {
@@ -589,19 +590,28 @@ namespace std
 #endif

 #if defined(_GLIBCXX_ASSERTIONS)
-# define _GLIBCXX_DO_ASSERT true
-#elif _GLIBCXX_HAVE_IS_CONSTANT_EVALUATED
-# define _GLIBCXX_DO_ASSERT std::__is_constant_evaluated()
-#else
-# define _GLIBCXX_DO_ASSERT false
-#endif
-
 # define __glibcxx_assert(cond)                                               
\
   do {                                                                 \
-    if _GLIBCXX17_CONSTEXPR (_GLIBCXX_DO_ASSERT)                       \
-      if (__builtin_expect(!bool(cond), false))                               
\
-       _GLIBCXX_ASSERT_FAIL(cond);                                     \
+    if (__builtin_expect(!bool(cond), false))                          \
+      _GLIBCXX_ASSERT_FAIL(cond);                                      \
   } while (false)
+#elif _GLIBCXX_HAVE_IS_CONSTANT_EVALUATED
+namespace std
+{
+  __attribute__((__always_inline__,visibility("default")))
+  inline void
+  __glibcxx_compile_time_assert_fail()
+  { }
+}
+# define __glibcxx_assert(cond)                                               
\
+  do {                                                                 \
+    if (std::__is_constant_evaluated())                                       
\
+      if (__builtin_expect(!bool(cond), false))                               
\
+       __glibcxx_compile_time_assert_fail();                           \
+  } while (false)
+#else
+# define __glibcxx_assert(cond)
+#endif

 // Macro indicating that TSAN is in use.
 #if __SANITIZE_THREAD__