[libcxx-commits] [libcxx] [libc++] Avoid code duplication in strings operator+ overloads (PR #126048)

[Louis Dionne via libcxx-commits]

================
@@ -3986,83 +3940,73 @@ operator>=(const _CharT* __lhs, const basic_string<_CharT, _Traits, _Allocator>&
 
 template <class _CharT, class _Traits, class _Allocator>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string<_CharT, _Traits, _Allocator>
-operator+(const basic_string<_CharT, _Traits, _Allocator>& __lhs,
-          const basic_string<_CharT, _Traits, _Allocator>& __rhs) {
+__concatenate_strings(const _Allocator& __alloc,
+                      __type_identity_t<basic_string_view<_CharT, _Traits> > __str1,
+                      __type_identity_t<basic_string_view<_CharT, _Traits> > __str2) {
   using _String = basic_string<_CharT, _Traits, _Allocator>;
-  auto __lhs_sz = __lhs.size();
-  auto __rhs_sz = __rhs.size();
   _String __r(__uninitialized_size_tag(),
-              __lhs_sz + __rhs_sz,
-              _String::__alloc_traits::select_on_container_copy_construction(__lhs.get_allocator()));
+              __str1.size() + __str2.size(),
+              _String::__alloc_traits::select_on_container_copy_construction(__alloc));
   auto __ptr = std::__to_address(__r.__get_pointer());
-  _Traits::copy(__ptr, __lhs.data(), __lhs_sz);
-  _Traits::copy(__ptr + __lhs_sz, __rhs.data(), __rhs_sz);
-  _Traits::assign(__ptr + __lhs_sz + __rhs_sz, 1, _CharT());
+  _Traits::copy(__ptr, __str1.data(), __str1.size());
+  _Traits::copy(__ptr + __str1.size(), __str2.data(), __str2.size());
+  _Traits::assign(__ptr[__str1.size() + __str2.size()], _CharT());
   return __r;
 }
 
+template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string<_CharT, _Traits, _Allocator>
+operator+(const basic_string<_CharT, _Traits, _Allocator>& __lhs,
+          const basic_string<_CharT, _Traits, _Allocator>& __rhs) {
+  return std::__concatenate_strings<_CharT, _Traits>(__lhs.get_allocator(), __lhs, __rhs);
----------------
ldionne wrote:

If you make a "mistake" and you use `__rhs.get_allocator()` here instead, does any test fail? If not, can we add coverage for that (unless both behaviors are conforming)?

https://github.com/llvm/llvm-project/pull/126048