[gcc r9-9424] libstdc++: Fix up constexpr std::char_traits<char>::compare [PR99181]

[Jakub Jelinek <jakub@gcc.gnu.org>]

https://gcc.gnu.org/g:03e18a40070c6fe89397a73d85a38a99371cf8a1

commit r9-9424-g03e18a40070c6fe89397a73d85a38a99371cf8a1
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Feb 23 09:30:18 2021 +0100

    libstdc++: Fix up constexpr std::char_traits<char>::compare [PR99181]
    
    Because of LWG 467, std::char_traits<char>::lt compares the values
    cast to unsigned char rather than char, so even when char is signed
    we get unsigned comparision.  std::char_traits<char>::compare uses
    __builtin_memcmp and that works the same, but during constexpr evaluation
    we were calling __gnu_cxx::char_traits<char_type>::compare.  As
    char_traits::lt is not virtual, __gnu_cxx::char_traits<char_type>::compare
    used __gnu_cxx::char_traits<char_type>::lt rather than
    std::char_traits<char>::lt and thus compared chars as signed if char is
    signed.
    This change fixes it by inlining __gnu_cxx::char_traits<char_type>::compare
    into std::char_traits<char>::compare by hand, so that it calls the right
    lt method.
    
    2021-02-23  Jakub Jelinek  <jakub@redhat.com>
    
            PR libstdc++/99181
            * include/bits/char_traits.h (char_traits<char>::compare): For
            constexpr evaluation don't call
            __gnu_cxx::char_traits<char_type>::compare but do the comparison loop
            directly.
    
            * testsuite/21_strings/char_traits/requirements/char/99181.cc: New
            test.
    
    (cherry picked from commit 311c57f6d8f285d69e44bf94152c753900cb1a0a)

Diff:
---
 libstdc++-v3/include/bits/char_traits.h            |  9 ++++-
 .../char_traits/requirements/char/99181.cc         | 40 ++++++++++++++++++++++
 2 files changed, 48 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/char_traits.h b/libstdc++-v3/include/bits/char_traits.h
index fd9a3c73930..50bd25eb2df 100644
--- a/libstdc++-v3/include/bits/char_traits.h
+++ b/libstdc++-v3/include/bits/char_traits.h
@@ -320,7 +320,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	if (__builtin_constant_p(__n)
 	    && __constant_char_array_p(__s1, __n)
 	    && __constant_char_array_p(__s2, __n))
-	  return __gnu_cxx::char_traits<char_type>::compare(__s1, __s2, __n);
+	  {
+	    for (size_t __i = 0; __i < __n; ++__i)
+	      if (lt(__s1[__i], __s2[__i]))
+		return -1;
+	      else if (lt(__s2[__i], __s1[__i]))
+		return 1;
+	    return 0;
+	  }
 #endif
 	return __builtin_memcmp(__s1, __s2, __n);
       }
diff --git a/libstdc++-v3/testsuite/21_strings/char_traits/requirements/char/99181.cc b/libstdc++-v3/testsuite/21_strings/char_traits/requirements/char/99181.cc
new file mode 100644
index 00000000000..01a70da046c
--- /dev/null
+++ b/libstdc++-v3/testsuite/21_strings/char_traits/requirements/char/99181.cc
@@ -0,0 +1,40 @@
+// { dg-options "-std=gnu++17" }
+// { dg-do run { target c++17 } }
+
+// Copyright (C) 2021 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>
+#include <testsuite_hooks.h>
+
+void test01()
+{
+  const char *a = "\x7f";
+  const char *b = "\x80";
+  int c = std::char_traits<char>::compare(a, b, 2);
+  constexpr int d = std::char_traits<char>::compare("\x7f", "\x80", 2);
+
+  VERIFY( c && (c < 0) == (static_cast<unsigned char>(a[0])
+			   < static_cast<unsigned char>(b[0])) );
+  VERIFY( d && (c < 0) == (d < 0) );
+}
+
+int main()
+{
+  test01();
+  return 0;
+}