[gcc r12-9331] libstdc++: More fixes for null pointers used with std::char_traits

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:19193bf7274fbba33f64d1892a5aa5be072e85cb

commit r12-9331-g19193bf7274fbba33f64d1892a5aa5be072e85cb
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Mar 28 11:12:58 2023 +0100

    libstdc++: More fixes for null pointers used with std::char_traits
    
    The std::char_traits member functions require that [p,p+n) is a valid
    range, which is true for p==nullptr iff n==0. But we must not call
    memcpy, memset etc, in that case, as they require non-null pointers even
    when n==0.
    
    This std::char_traits<char> and std::char_traits<wchar_t> explicit
    specializations are already correct, but the primary template has some
    bugs.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/char_traits.h (char_traits::copy): Return without
            using memcpy if n==0.
            (char_traits::assign): Likewise for memset.
    
    (cherry picked from commit cb6f663f9d79d7134ae6ecaff9a25342c40aeb5d)

Diff:
---
 libstdc++-v3/include/bits/char_traits.h | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/bits/char_traits.h b/libstdc++-v3/include/bits/char_traits.h
index cac1326d4b5..3c136f930ef 100644
--- a/libstdc++-v3/include/bits/char_traits.h
+++ b/libstdc++-v3/include/bits/char_traits.h
@@ -255,6 +255,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     char_traits<_CharT>::
     copy(char_type* __s1, const char_type* __s2, std::size_t __n)
     {
+      if (__n == 0)
+	return __s1;
 #if __cplusplus >= 202002L
       if (std::__is_constant_evaluated())
 	{
@@ -263,7 +265,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  return __s1;
 	}
 #endif
-
       __builtin_memcpy(__s1, __s2, __n * sizeof(char_type));
       return __s1;
     }
@@ -285,9 +286,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       if _GLIBCXX17_CONSTEXPR (sizeof(_CharT) == 1 && __is_trivial(_CharT))
 	{
-	  unsigned char __c;
-	  __builtin_memcpy(&__c, __builtin_addressof(__a), 1);
-	  __builtin_memset(__s, __c, __n);
+	  if (__n)
+	    {
+	      unsigned char __c;
+	      __builtin_memcpy(&__c, __builtin_addressof(__a), 1);
+	      __builtin_memset(__s, __c, __n);
+	    }
 	}
       else
 	{