Try it again. Use the approach suggested by Tim in the LWG thread :
using function default argument SFINAE

- Revert "[libc++] Revert LWG3233 Broken requirements for shared_ptr
converting constructors (#93071)"
- Revert "[libc++] Revert temporary attempt to implement LWG 4110
(#95263)"
- test for default_delete
- Revert "Revert "[libc++] Revert temporary attempt to implement LWG
4110 (#95263)""
- test for NULL

Fix several bugs:
1. https://llvm.org/PR60258
   The conversion constructors' constraint `__compatible_with` incorrectly allow array types conversion to scalar types
2. https://llvm.org/PR53368
   The constructor that takes `unique_ptr` are not suffiently constrained.
3. The constructors that take raw pointers incorretly use `__compatible_with`. They have different constraints

Differential Revision: https://reviews.llvm.org/D143346

Those tests were extracted from D120996.

Differential Revision: https://reviews.llvm.org/D121340

This reverts commit 276ca873. That commit has quite a history at this
point. It was first landed in dbc647643577, which broke std::shared_ptr<T const>
and was reverted in 9138666f5. It was then re-applied in 276ca873, with
the std::shared_ptr issue fixed, but it caused widespread breakage at
Google (which suggests it would cause similar breakage in the wild too),
so now I'm reverting again.

Instead, I will add a escape hatch that vendors can turn on to enable
the extension and perform a phased transition over one or two releases
like we sometimes do when things become non-trivial.

This extension is a portability trap for users, since no other standard
library supports it. Furthermore, the Standard explicitly allows
implementations to reject std::allocator<cv T>, so allowing it is
really going against the current.

This was discovered in D120684: this extension required `const_cast`ing
in `__construct_range_forward`, a fishy bit of code that can be removed
if we don't support the extension anymore.

This is a re-application of dbc647643577, which was reverted in 9138666f5
because it broke std::shared_ptr<T const>. Tests have now been added and
we've made sure that std::shared_ptr<T const> wouldn't be broken in this
version.

Differential Revision: https://reviews.llvm.org/D120996

The only possible kind of a conversion in initialization of a shared
pointer to an array is a qualification conversion (i.e., adding
cv-qualifiers). This patch adds tests for converting from `A[]` to
`const A[]` to the following functions:

```
template<class Y> explicit shared_ptr(Y* p);

template<class Y> shared_ptr(const shared_ptr<Y>& r);
template<class Y> shared_ptr(shared_ptr<Y>&& r);

template<class Y> shared_ptr& operator=(const shared_ptr<Y>& r);
template<class Y> shared_ptr& operator=(shared_ptr<Y>&& r);

template<class Y> void reset(Y* p);
template<class Y, class D> void reset(Y* p, D d);
template<class Y, class D, class A> void reset(Y* p, D d, A a);
```

Similar tests for converting functions that involve a `weak_ptr` should
be added once LWG issue [3001](https://cplusplus.github.io/LWG/issue3001)
is implemented.

Differential Revision: https://reviews.llvm.org/D112048

Implements P0414R2:
  * Adds support for array types in std::shared_ptr.
  * Adds reinterpret_pointer_cast for shared_ptr.

Re-committing now that the leaking tests are fixed.

Differential Revision: https://reviews.llvm.org/D62259

This reverts commit e8c13c182a562f45287d6b8da612264d09027087.

Implements P0414R2:
  * Adds support for array types in std::shared_ptr.
  * Adds reinterpret_pointer_cast for shared_ptr.

Differential Revision: https://reviews.llvm.org/D62259

llvm-svn: 362252