[libcxx-commits] [libcxx] [libc++] Add container_traits (prework for `std::flat_map`) (PR #109578)

[Louis Dionne via libcxx-commits]

================
@@ -3005,6 +3007,19 @@ public:
 };
 #endif // _LIBCPP_STD_VER >= 23
 
+template <class _Tp, class _Allocator>
+struct __container_traits<vector<_Tp, _Allocator> > {
+  // http://eel.is/c++draft/vector.modifiers#2
+  //  If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move
+  //  assignment operator of T or by any InputIterator operation, there are no effects. If an exception is thrown while
+  //  inserting a single element at the end and T is Cpp17CopyInsertable or is_nothrow_move_constructible_v<T> is true,
+  //  there are no effects. Otherwise, if an exception is thrown by the move constructor of a non-Cpp17CopyInsertable T,
+  //  the effects are unspecified.
+  using __emplacement_has_strong_exception_safety_guarantee =
+      _Or<is_nothrow_move_constructible<_Tp>,
+          __is_cpp17_copy_insertable<typename vector<_Tp, _Allocator>::allocator_type> >;
----------------
ldionne wrote:

```suggestion
          __is_cpp17_copy_insertable<_Allocator> >;
```

https://github.com/llvm/llvm-project/pull/109578