libstdc++: Fix char_traits move with overlap

Upon constexpr evaluation, char_traits move uses copy_backward, but its
last argument should be to the range end rather than its beginning.

2020-06-12  Paul Keir  <paul.keir@uws.ac.uk>

	* include/bits/char_traits.h (char_traits::move): constexpr move with
	overlap was using copy_backward incorrectly.
	* testsuite/21_strings/char_traits/requirements/constexpr_functions_c++20.cc:
	New test.

2 files changed, 53 insertions, 1 deletions

diff --git a/libstdc++-v3/include/bits/char_traits.h b/libstdc++-v3/include/bits/char_traits.h
index c6da184..c623a67 100644
--- a/libstdc++-v3/include/bits/char_traits.h
+++ b/libstdc++-v3/include/bits/char_traits.h
@@ -196,7 +196,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       if (std::is_constant_evaluated())
 	{
 	  if (__s1 > __s2 && __s1 < __s2 + __n)
-	    std::copy_backward(__s2, __s2 + __n, __s1);
+	    std::copy_backward(__s2, __s2 + __n, __s1 + __n);
 	  else
 	    std::copy(__s2, __s2 + __n, __s1);
 	  return __s1;
diff --git a/libstdc++-v3/testsuite/21_strings/char_traits/requirements/constexpr_functions_c++20.cc b/libstdc++-v3/testsuite/21_strings/char_traits/requirements/constexpr_functions_c++20.cc
new file mode 100644
index 0000000..6358640
--- /dev/null
+++ b/libstdc++-v3/testsuite/21_strings/char_traits/requirements/constexpr_functions_c++20.cc
@@ -0,0 +1,52 @@
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+// Copyright (C) 2017-2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+#include <string>
+
+template<typename CT>
+  constexpr bool
+  test_move()
+  {
+    using char_type = typename CT::char_type;
+    char_type s1[3] = {1, 2, 3};
+    CT::move(s1+1, s1, 2);
+    return s1[0]==char_type{1} && s1[1]==char_type{1} && s1[2]==char_type{2};
+  }
+
+#ifndef __cpp_lib_constexpr_char_traits
+# error Feature-test macro for constexpr char_traits is missing
+#elif __cpp_lib_constexpr_char_traits != 201611
+# error Feature-test macro for constexpr char_traits has the wrong value
+#endif
+
+static_assert( test_move<std::char_traits<char>>() );
+#ifdef _GLIBCXX_USE_WCHAR_T
+static_assert( test_move<std::char_traits<wchar_t>>() );
+#endif
+#ifdef _GLIBCXX_USE_CHAR8_T
+static_assert( test_move<std::char_traits<char8_t>>() );
+#endif
+static_assert( test_move<std::char_traits<char16_t>>() );
+static_assert( test_move<std::char_traits<char32_t>>() );
+
+struct C { unsigned char c; };
+constexpr bool operator==(const C& c1, const C& c2) { return c1.c == c2.c; }
+constexpr bool operator<(const C& c1, const C& c2) { return c1.c < c2.c; }
+static_assert( test_move<std::char_traits<C>>() );