From 26a9e8cee4d20e5b08c0336439c8f69a2f06af1c Mon Sep 17 00:00:00 2001
From: Jonathan Wakely <jwakely@redhat.com>
Date: Wed, 3 Jan 2024 15:01:09 +0000
Subject: [PATCH] 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.

(cherry picked from commit 15cc291887dc9dd92b2c93f4545e20eb6c190122)
---
 libstdc++-v3/include/bits/char_traits.h       | 16 +++------------
 .../char_traits/requirements/113200.cc        | 20 +++++++++++++++++++
 2 files changed, 23 insertions(+), 13 deletions(-)
 create mode 100644 libstdc++-v3/testsuite/21_strings/char_traits/requirements/113200.cc

diff --git a/libstdc++-v3/include/bits/char_traits.h b/libstdc++-v3/include/bits/char_traits.h
index 3c136f930ef8..a3a5bbfb6439 100644
--- a/libstdc++-v3/include/bits/char_traits.h
+++ b/libstdc++-v3/include/bits/char_traits.h
@@ -210,8 +210,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #if __cplusplus >= 202002L
       if (std::__is_constant_evaluated())
 	{
-	  if (__s1 == __s2) // unlikely, but saves a lot of work
-	    return __s1;
 #if __cpp_constexpr_dynamic_alloc
 	  // The overlap detection below fails due to PR c++/89074,
 	  // so use a temporary buffer instead.
@@ -220,17 +218,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  copy(__s1, __tmp, __n);
 	  delete[] __tmp;
 #else
-	  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
 		{
diff --git a/libstdc++-v3/testsuite/21_strings/char_traits/requirements/113200.cc b/libstdc++-v3/testsuite/21_strings/char_traits/requirements/113200.cc
new file mode 100644
index 000000000000..0fe765d53bcc
--- /dev/null
+++ b/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" );
-- 
2.47.2