libstdc++: Make string_view::copy usable in constant expressions (PR 94498)

	PR libstdc++/94498
	* include/bits/char_traits.h (__gnu_cxx::char_traits::move): Make it
	usable in constant expressions for C++20.
	(__gnu_cxx::char_traits::copy, __gnu_cxx::char_traits::assign): Add
	_GLIBCXX20_CONSTEXPR.
	(std::char_traits<char>, std::char_traits<wchar_t>)
	(std::char_traits<char8_t>): Make move, copy and assign usable in
	constant expressions for C++20.
	(std::char_traits<char16_t>, std::char_traits<char32_t>): Make move
	and copy usable in constant expressions for C++20.
	* include/std/string_view (basic_string_view::copy): Add
	_GLIBCXX20_CONSTEXPR.
	* testsuite/21_strings/basic_string_view/operations/copy/char/
	constexpr.cc: New test.
	* testsuite/21_strings/basic_string_view/operations/copy/wchar_t/
	constexpr.cc: New test.

2 files changed, 84 insertions, 18 deletions

diff --git a/libstdc++-v3/include/bits/char_traits.h b/libstdc++-v3/include/bits/char_traits.h
index 2edb84ca3b8..65031d3ac83 100644
--- a/libstdc++-v3/include/bits/char_traits.h
+++ b/libstdc++-v3/include/bits/char_traits.h
@@ -113,13 +113,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       static _GLIBCXX14_CONSTEXPR const char_type*
       find(const char_type* __s, std::size_t __n, const char_type& __a);
 
-      static char_type*
+      static _GLIBCXX20_CONSTEXPR char_type*
       move(char_type* __s1, const char_type* __s2, std::size_t __n);
 
-      static char_type*
+      static _GLIBCXX20_CONSTEXPR char_type*
       copy(char_type* __s1, const char_type* __s2, std::size_t __n);
 
-      static char_type*
+      static _GLIBCXX20_CONSTEXPR char_type*
       assign(char_type* __s, std::size_t __n, char_type __a);
 
       static _GLIBCXX_CONSTEXPR char_type
@@ -179,17 +179,29 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT>
+    _GLIBCXX20_CONSTEXPR
     typename char_traits<_CharT>::char_type*
     char_traits<_CharT>::
     move(char_type* __s1, const char_type* __s2, std::size_t __n)
     {
       if (__n == 0)
 	return __s1;
+#ifdef __cpp_lib_is_constant_evaluated
+      if (std::is_constant_evaluated())
+	{
+	  if (__s1 > __s2 && __s1 < __s2 + __n)
+	    std::copy_backward(__s2, __s2 + __n, __s1);
+	  else
+	    std::copy(__s2, __s2 + __n, __s1);
+	  return __s1;
+	}
+#endif
       return static_cast<_CharT*>(__builtin_memmove(__s1, __s2,
 						    __n * sizeof(char_type)));
     }
 
   template<typename _CharT>
+    _GLIBCXX20_CONSTEXPR
     typename char_traits<_CharT>::char_type*
     char_traits<_CharT>::
     copy(char_type* __s1, const char_type* __s2, std::size_t __n)
@@ -200,6 +212,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT>
+    _GLIBCXX20_CONSTEXPR
     typename char_traits<_CharT>::char_type*
     char_traits<_CharT>::
     assign(char_type* __s, std::size_t __n, char_type __a)
@@ -349,27 +362,39 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	return static_cast<const char_type*>(__builtin_memchr(__s, __a, __n));
       }
 
-      static char_type*
+      static _GLIBCXX20_CONSTEXPR char_type*
       move(char_type* __s1, const char_type* __s2, size_t __n)
       {
 	if (__n == 0)
 	  return __s1;
+#ifdef __cpp_lib_is_constant_evaluated
+	if (std::is_constant_evaluated())
+	  return __gnu_cxx::char_traits<char_type>::move(__s1, __s2, __n);
+#endif
 	return static_cast<char_type*>(__builtin_memmove(__s1, __s2, __n));
       }
 
-      static char_type*
+      static _GLIBCXX20_CONSTEXPR char_type*
       copy(char_type* __s1, const char_type* __s2, size_t __n)
       {
 	if (__n == 0)
 	  return __s1;
+#ifdef __cpp_lib_is_constant_evaluated
+	if (std::is_constant_evaluated())
+	  return __gnu_cxx::char_traits<char_type>::copy(__s1, __s2, __n);
+#endif
 	return static_cast<char_type*>(__builtin_memcpy(__s1, __s2, __n));
       }
 
-      static char_type*
+      static _GLIBCXX20_CONSTEXPR char_type*
       assign(char_type* __s, size_t __n, char_type __a)
       {
 	if (__n == 0)
 	  return __s;
+#ifdef __cpp_lib_is_constant_evaluated
+	if (std::is_constant_evaluated())
+	  return __gnu_cxx::char_traits<char_type>::assign(__s, __n, __a);
+#endif
 	return static_cast<char_type*>(__builtin_memset(__s, __a, __n));
       }
 
@@ -458,27 +483,39 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	return wmemchr(__s, __a, __n);
       }
 
-      static char_type*
+      static _GLIBCXX20_CONSTEXPR char_type*
       move(char_type* __s1, const char_type* __s2, size_t __n)
       {
 	if (__n == 0)
 	  return __s1;
+#ifdef __cpp_lib_is_constant_evaluated
+	if (std::is_constant_evaluated())
+	  return __gnu_cxx::char_traits<char_type>::move(__s1, __s2, __n);
+#endif
 	return wmemmove(__s1, __s2, __n);
       }
 
-      static char_type*
+      static _GLIBCXX20_CONSTEXPR char_type*
       copy(char_type* __s1, const char_type* __s2, size_t __n)
       {
 	if (__n == 0)
 	  return __s1;
+#ifdef __cpp_lib_is_constant_evaluated
+	if (std::is_constant_evaluated())
+	  return __gnu_cxx::char_traits<char_type>::copy(__s1, __s2, __n);
+#endif
 	return wmemcpy(__s1, __s2, __n);
       }
 
-      static char_type*
+      static _GLIBCXX20_CONSTEXPR char_type*
       assign(char_type* __s, size_t __n, char_type __a)
       {
 	if (__n == 0)
 	  return __s;
+#ifdef __cpp_lib_is_constant_evaluated
+	if (std::is_constant_evaluated())
+	  return __gnu_cxx::char_traits<char_type>::assign(__s, __n, __a);
+#endif
 	return wmemset(__s, __a, __n);
       }
 
@@ -567,27 +604,39 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	return static_cast<const char_type*>(__builtin_memchr(__s, __a, __n));
       }
 
-      static char_type*
+      static _GLIBCXX20_CONSTEXPR char_type*
       move(char_type* __s1, const char_type* __s2, size_t __n)
       {
 	if (__n == 0)
 	  return __s1;
+#ifdef __cpp_lib_is_constant_evaluated
+	if (std::is_constant_evaluated())
+	  return __gnu_cxx::char_traits<char_type>::move(__s1, __s2, __n);
+#endif
 	return static_cast<char_type*>(__builtin_memmove(__s1, __s2, __n));
       }
 
-      static char_type*
+      static _GLIBCXX20_CONSTEXPR char_type*
       copy(char_type* __s1, const char_type* __s2, size_t __n)
       {
 	if (__n == 0)
 	  return __s1;
+#ifdef __cpp_lib_is_constant_evaluated
+	if (std::is_constant_evaluated())
+	  return __gnu_cxx::char_traits<char_type>::copy(__s1, __s2, __n);
+#endif
 	return static_cast<char_type*>(__builtin_memcpy(__s1, __s2, __n));
       }
 
-      static char_type*
+      static _GLIBCXX20_CONSTEXPR char_type*
       assign(char_type* __s, size_t __n, char_type __a)
       {
 	if (__n == 0)
 	  return __s;
+#ifdef __cpp_lib_is_constant_evaluated
+	if (std::is_constant_evaluated())
+	  return __gnu_cxx::char_traits<char_type>::assign(__s, __n, __a);
+#endif
 	return static_cast<char_type*>(__builtin_memset(__s, __a, __n));
       }
 
@@ -680,25 +729,33 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	return 0;
       }
 
-      static char_type*
+      static _GLIBCXX20_CONSTEXPR char_type*
       move(char_type* __s1, const char_type* __s2, size_t __n)
       {
 	if (__n == 0)
 	  return __s1;
+#ifdef __cpp_lib_is_constant_evaluated
+	if (std::is_constant_evaluated())
+	  return __gnu_cxx::char_traits<char_type>::move(__s1, __s2, __n);
+#endif
 	return (static_cast<char_type*>
 		(__builtin_memmove(__s1, __s2, __n * sizeof(char_type))));
       }
 
-      static char_type*
+      static _GLIBCXX20_CONSTEXPR char_type*
       copy(char_type* __s1, const char_type* __s2, size_t __n)
       {
 	if (__n == 0)
 	  return __s1;
+#ifdef __cpp_lib_is_constant_evaluated
+	if (std::is_constant_evaluated())
+	  return __gnu_cxx::char_traits<char_type>::copy(__s1, __s2, __n);
+#endif
 	return (static_cast<char_type*>
 		(__builtin_memcpy(__s1, __s2, __n * sizeof(char_type))));
       }
 
-      static char_type*
+      static _GLIBCXX20_CONSTEXPR char_type*
       assign(char_type* __s, size_t __n, char_type __a)
       {
 	for (size_t __i = 0; __i < __n; ++__i)
@@ -783,25 +840,33 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	return 0;
       }
 
-      static char_type*
+      static _GLIBCXX20_CONSTEXPR char_type*
       move(char_type* __s1, const char_type* __s2, size_t __n)
       {
 	if (__n == 0)
 	  return __s1;
+#ifdef __cpp_lib_is_constant_evaluated
+	if (std::is_constant_evaluated())
+	  return __gnu_cxx::char_traits<char_type>::move(__s1, __s2, __n);
+#endif
 	return (static_cast<char_type*>
 		(__builtin_memmove(__s1, __s2, __n * sizeof(char_type))));
       }
 
-      static char_type*
+      static _GLIBCXX20_CONSTEXPR char_type*
       copy(char_type* __s1, const char_type* __s2, size_t __n)
       { 
 	if (__n == 0)
 	  return __s1;
+#ifdef __cpp_lib_is_constant_evaluated
+	if (std::is_constant_evaluated())
+	  return __gnu_cxx::char_traits<char_type>::copy(__s1, __s2, __n);
+#endif
 	return (static_cast<char_type*>
 		(__builtin_memcpy(__s1, __s2, __n * sizeof(char_type))));
       }
 
-      static char_type*
+      static _GLIBCXX20_CONSTEXPR char_type*
       assign(char_type* __s, size_t __n, char_type __a)
       {
 	for (size_t __i = 0; __i < __n; ++__i)
diff --git a/libstdc++-v3/include/std/string_view b/libstdc++-v3/include/std/string_view
index 99a81bb04fa..ea1ccc9bb21 100644
--- a/libstdc++-v3/include/std/string_view
+++ b/libstdc++-v3/include/std/string_view
@@ -263,6 +263,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       // [string.view.ops], string operations:
 
+      _GLIBCXX20_CONSTEXPR
       size_type
       copy(_CharT* __str, size_type __n, size_type __pos = 0) const
       {