[committed] libstdc++: More fixes for null pointers used with std::char_traits

[Jonathan Wakely via Gcc-patches]

Tested powerpc64le-linux. Pushed to trunk.

-- >8 --

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.
---
 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 cfc7d0f0e14..68ed827f982 100644
--- a/libstdc++-v3/include/bits/char_traits.h
+++ b/libstdc++-v3/include/bits/char_traits.h
@@ -263,6 +263,8 @@ _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())
        {
@@ -271,7 +273,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
          return __s1;
        }
 #endif
-
       __builtin_memcpy(__s1, __s2, __n * sizeof(char_type));
       return __s1;
     }
@@ -293,9 +294,12 @@ _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
        {
-- 
2.39.2