[libc++] Deprecates and removes shared_ptr::unqiue. (#76576)

The status table incorrectly marks P0521R0 as nothing to do. This is not
correct the function should be deprecated.
During our latest monthly meeting we argreed to remove the
_LIBCPP_ENABLE_CXXyy_REMOVED_FEATURES macros, therefore the new macro is
not
added to that global list.

Implements
- P0521R0 Proposed Resolution for CA 14 (shared_ptr use_count/unique)

Implements parts of
- P0619R4 Reviewing Deprecated Facilities of C++17 for C++20

---------

Co-authored-by: Nikolas Klauser <nikolasklauser@berlin.de>

libcxx/docs/ReleaseNotes/18.rst

@@ -57,6 +57,7 @@ Implemented Papers
 - P2871R3 - Remove Deprecated Unicode Conversion Facets from C++26
 - P2870R3 - Remove basic_string::reserve()
 - P2909R4 - Fix formatting of code units as integers (Dude, where’s my ``char``?)
+- P0521R0 - Proposed Resolution for CA 14 (shared_ptr use_count/unique)
 
 
 Improvements and New Features
@@ -83,6 +84,9 @@ Improvements and New Features
 - The ``_LIBCPP_ENABLE_CXX26_REMOVED_STRING_RESERVE`` macro has been added to make
   the function ``std::basic_string<...>::reserve()`` available.
 
+- The ``_LIBCPP_ENABLE_CXX20_REMOVED_SHARED_PTR_UNIQUE`` macro has been added to make
+  the function ``std::shared_ptr<...>::unique()`` available.
+
 
 Deprecations and Removals
 -------------------------

libcxx/docs/Status/Cxx17Papers.csv

@@ -87,7 +87,7 @@
 "`P0513R0 <https://wg21.link/P0513R0>`__","LWG","Poisoning the Hash","Issaquah","|Complete|","5.0"
 "`P0516R0 <https://wg21.link/P0516R0>`__","LWG","Clarify That shared_future's Copy Operations have Wide Contracts","Issaquah","|Complete|","4.0"
 "`P0517R0 <https://wg21.link/P0517R0>`__","LWG","Make future_error Constructible","Issaquah","|Complete|","4.0"
-"`P0521R0 <https://wg21.link/P0521R0>`__","LWG","Proposed Resolution for CA 14 (shared_ptr use_count/unique)","Issaquah","|Nothing To Do|","n/a"
+"`P0521R0 <https://wg21.link/P0521R0>`__","LWG","Proposed Resolution for CA 14 (shared_ptr use_count/unique)","Issaquah","|Complete|","18.0"
 "","","","","",""
 "`P0156R2 <https://wg21.link/P0156R2>`__","LWG","Variadic Lock guard(rev 5)","Kona","|Complete|","5.0"
 "`P0270R3 <https://wg21.link/P0270R3>`__","CWG","Removing C dependencies from signal handler wording","Kona","",""

libcxx/docs/Status/Cxx20.rst

@@ -44,7 +44,7 @@ Paper Status
    .. [#note-P0645] P0645: The paper is implemented but still marked as an incomplete feature
       (the feature-test macro is not set).
    .. [#note-P0966] P0966: It was previously erroneously marked as complete in version 8.0. See `bug 45368 <https://llvm.org/PR45368>`__.
-   .. [#note-P0619] P0619: Only sections D.8, D.9, D.10 and D.13 are implemented. Sections D.4, D.7, D.11, D.12, and D.14 remain undone.
+   .. [#note-P0619] P0619: Only sections D.8, D.9, D.10 and D.13 are implemented. Sections D.4, D.7, D.11, and D.12 remain undone.
    .. [#note-P0883.1] P0883: shared_ptr and floating-point changes weren't applied as they themselves aren't implemented yet.
    .. [#note-P0883.2] P0883: ``ATOMIC_FLAG_INIT`` was marked deprecated in version 14.0, but was undeprecated with the implementation of LWG3659 in version 15.0.
    .. [#note-P2231] P2231: Optional is complete. The changes to variant haven't been implemented yet.

libcxx/docs/UsingLibcxx.rst

@@ -296,6 +296,10 @@ C++17 Specific Configuration Macros
 
 C++20 Specific Configuration Macros
 -----------------------------------
+**_LIBCPP_ENABLE_CXX20_REMOVED_SHARED_PTR_UNIQUE**
+  This macro is used to re-enable the function
+  ``std::shared_ptr<...>::unique()``.
+
 **_LIBCPP_ENABLE_CXX20_REMOVED_FEATURES**:
   This macro is used to re-enable all the features removed in C++20. The effect
   is equivalent to manually defining each macro listed below.

libcxx/include/__memory/shared_ptr.h

@@ -723,7 +723,9 @@ public:
 
   _LIBCPP_HIDE_FROM_ABI long use_count() const _NOEXCEPT { return __cntrl_ ? __cntrl_->use_count() : 0; }
 
-  _LIBCPP_HIDE_FROM_ABI bool unique() const _NOEXCEPT { return use_count() == 1; }
+#if _LIBCPP_STD_VER < 20 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_SHARED_PTR_UNIQUE)
+  _LIBCPP_DEPRECATED_IN_CXX17 _LIBCPP_HIDE_FROM_ABI bool unique() const _NOEXCEPT { return use_count() == 1; }
+#endif
 
   _LIBCPP_HIDE_FROM_ABI explicit operator bool() const _NOEXCEPT { return get() != nullptr; }
 

libcxx/include/memory

@@ -629,7 +629,7 @@ public:
     T& operator*() const noexcept;
     T* operator->() const noexcept;
     long use_count() const noexcept;
-    bool unique() const noexcept;
+    bool unique() const noexcept;  // deprected in C++17, removed in C++20
     explicit operator bool() const noexcept;
     template<class U> bool owner_before(shared_ptr<U> const& b) const noexcept;
     template<class U> bool owner_before(weak_ptr<U> const& b) const noexcept;

libcxx/test/std/utilities/memory/util.smartptr/util.smartptr.shared/util.smartptr.shared.obs/unique.deprecated_in_cxx17.verify.cpp

@@ -0,0 +1,24 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14
+
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_CXX20_REMOVED_SHARED_PTR_UNIQUE
+
+// <memory>
+
+// shared_ptr
+
+// bool unique() const; // deprecated in C++17, removed in C++20
+
+#include <memory>
+
+void f() {
+  const std::shared_ptr<int> p;
+  p.unique(); // expected-warning {{'unique' is deprecated}}
+}

libcxx/test/std/utilities/memory/util.smartptr/util.smartptr.shared/util.smartptr.shared.obs/unique.pass.cpp

@@ -10,7 +10,9 @@
 
 // shared_ptr
 
-// bool unique() const;
+// bool unique() const; // deprecated in C++17, removed in C++20
+
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS -D_LIBCPP_ENABLE_CXX20_REMOVED_SHARED_PTR_UNIQUE
 
 #include <memory>
 #include <cassert>

libcxx/test/std/utilities/memory/util.smartptr/util.smartptr.shared/util.smartptr.shared.obs/unique.removed_in_cxx20.verify.cpp

@@ -0,0 +1,22 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// <memory>
+
+// shared_ptr
+
+// bool unique() const; // deprecated in C++17, removed in C++20
+
+#include <memory>
+
+void f() {
+  const std::shared_ptr<int> p;
+  p.unique(); // expected-error {{no member named 'unique' in 'std::shared_ptr<int>}}
+}