[libc++] Implemented make_unique_for_overwrite

[ldionne]

Summary

Implemented std::make_unique_for_overwrite according to the following paper: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1020r1.html
and the rename http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1973r1.pdf

[ahmadbeirkdar]

Typo in _LIBCPP_STD_VER check, and an extra semi colon.

[curdeius]

There's already an ongoing effort to implement this: D90111.
You might want to sync with the author of the other patch.

[ahmadbeirkdar]

In D93590#2464702, @curdeius wrote:

There's already an ongoing effort to implement this: D90111.
You might want to sync with the author of the other patch.

Oh thanks, sorry did not know. First time trying to contribute over this platform and to libcxx

[ldionne]

Supersedded by D140913, closing.

libcxx/include/memory

 template<class T>                unique_ptr<T> make_unique(size_t n);           // C++14
 template<class T, class... Args> unspecified   make_unique(Args&&...) = delete; // C++14, T == U[N]
 
+template<class T>                unique_ptr<T> make_unique_for_overwrite();                   // C++20, T not an array
+template<class T>                unique_ptr<T> make_unique_for_overwrite(size_t n);           // C++20, T == U[]
+template<class T, class... Args> unspecified   make_unique_for_overwrite(Args&&...) = delete; // C++20, T == U[N]
+
 template<class E, class T, class Y, class D>
     basic_ostream<E, T>& operator<< (basic_ostream<E, T>& os, unique_ptr<Y, D> const& p);
 

 
 #endif  // _LIBCPP_STD_VER > 11
 
+#if _LIBCPP_STD_VER >= 20
+
+template<class _Tp>
+inline _LIBCPP_INLINE_VISIBILITY
+typename __unique_if<_Tp>::__unique_single
+make_unique_for_overwrite()
+{
+    return unique_ptr<_Tp>(new _Tp);
+}
+
+template<class _Tp>
+inline _LIBCPP_INLINE_VISIBILITY
+typename __unique_if<_Tp>::__unique_array_unknown_bound
+make_unique_for_overwrite(size_t __n)
+{
+    typedef typename remove_extent<_Tp>::type _Up;
+    return unique_ptr<_Tp>(new _Up[__n]);
+}
+
+template<class _Tp, class... _Args>
+    typename __unique_if<_Tp>::__unique_array_known_bound
+    make_unique_for_overwrite(_Args&&...) = delete;
+
+#endif // _LIBCPP_STD_VER >= 20
+
 template <class _Tp, class _Dp>
 #ifdef _LIBCPP_CXX03_LANG
 struct _LIBCPP_TEMPLATE_VIS hash<unique_ptr<_Tp, _Dp> >