[libcxx-commits] [libcxx] WIP - [libc++][strings] P2591R5: Concatenation of strings and string views (PR #88389)

[A. Jiang via libcxx-commits]

================
@@ -4005,6 +4023,51 @@ operator+(basic_string<_CharT, _Traits, _Allocator>&& __lhs, _CharT __rhs) {
 
 #endif // _LIBCPP_CXX03_LANG
 
+#if _LIBCPP_STD_VER >= 26
+
+template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_HIDE_FROM_ABI constexpr basic_string<_CharT, _Traits, _Allocator>
+operator+(const basic_string<_CharT, _Traits, _Allocator>& __lhs,
+          type_identity_t<basic_string_view<_CharT, _Traits>> __rhs) {
+  using _String = basic_string<_CharT, _Traits, _Allocator>;
+
+  _String __r = __lhs;
+  __r.append(__rhs);
+  return __r;
+}
+
+template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_HIDE_FROM_ABI constexpr basic_string<_CharT, _Traits, _Allocator>
+operator+(basic_string<_CharT, _Traits, _Allocator>&& __lhs,
+          type_identity_t<basic_string_view<_CharT, _Traits>> __rhs) {
+  __lhs.append(__rhs);
+  return std::move(__lhs);
+}
+
+template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_HIDE_FROM_ABI constexpr basic_string<_CharT, _Traits, _Allocator>
+operator+(type_identity_t<basic_string_view<_CharT, _Traits>> __lhs,
+          const basic_string<_CharT, _Traits, _Allocator>& __rhs) {
+  using _String = basic_string<_CharT, _Traits, _Allocator>;
+
+  _String __r{__lhs};
+  __r.append(__rhs);
+  return __r;
----------------
frederick-vs-ja wrote:

The current approach will fail if `_Allocator` is not default-constructible, because the allocator is constructed by `_Allocator()` (ditto below). I think it's perfectly fine to follow the code shown in the paper.
```suggestion
  auto __r = __rhs;
  __r.insert(0, __lhs);
  return __r;
```

https://github.com/llvm/llvm-project/pull/88389