libstdc++: Fix std::char_traits<C>::move [PR113200]

The current constexpr implementation of std::char_traits<C>::move relies
on being able to compare the pointer parameters, which is not allowed
for unrelated pointers. We can use __builtin_constant_p to determine
whether it's safe to compare the pointers directly. If not, then we know
the ranges must be disjoint and so we can use char_traits<C>::copy to
copy forwards from the first character to the last. If the pointers can
be compared directly, then we can simplify the condition for copying
backwards to just two pointer comparisons.

libstdc++-v3/ChangeLog:

	PR libstdc++/113200
	* include/bits/char_traits.h (__gnu_cxx::char_traits::move): Use
	__builtin_constant_p to check for unrelated pointers that cannot
	be compared during constant evaluation.
	* testsuite/21_strings/char_traits/requirements/113200.cc: New
	test.

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

@@ -227,19 +227,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #if __cplusplus >= 202002L
       if (std::__is_constant_evaluated())
 	{
-	  if (__s1 == __s2) // unlikely, but saves a lot of work
-	    return __s1;
-	  const auto __end = __s2 + __n - 1;
-	  bool __overlap = false;
-	  for (std::size_t __i = 0; __i < __n - 1; ++__i)
-	    {
-	      if (__s1 + __i == __end)
-		{
-		  __overlap = true;
-		  break;
-		}
-	    }
-	  if (__overlap)
+	  // Use __builtin_constant_p to avoid comparing unrelated pointers.
+	  if (__builtin_constant_p(__s2 < __s1)
+		&& __s1 > __s2 && __s1 < (__s2 + __n))
 	    {
 	      do
 		{

libstdc++-v3/testsuite/21_strings/char_traits/requirements/113200.cc

@@ -0,0 +1,20 @@
+// { dg-do compile { target c++20 } }
+
+// PR libstdc++/113200
+// char_traits::move is not constexpr when the argument is a string literal
+
+#include <string_view>
+
+template<std::size_t N> struct S
+{
+  char data_[ N ];
+
+  constexpr S( char const* p ): data_{}
+  {
+    std::char_traits<char>::move( data_, p, N );
+  }
+};
+
+template<std::size_t N> S( char const(&)[N] ) -> S<N>;
+
+constexpr S s( "test" );