[libc++] Removes basic_string::reserve(). (#73354)

Implements:
- P2870R3 Remove basic_string::reserve()

---------

Co-authored-by: philnik777 <nikolasklauser@berlin.de>

libcxx/docs/ReleaseNotes/18.rst

@@ -52,6 +52,7 @@ Implemented Papers
 - P0020R6 - Floating Point Atomic
 - P2918R2 - Runtime format strings II
 - P2871R3 - Remove Deprecated Unicode Conversion Facets from C++26
+- P2870R3 - Remove basic_string::reserve()
 
 
 Improvements and New Features
@@ -75,6 +76,9 @@ Improvements and New Features
 - The ``_LIBCPP_ENABLE_CXX26_REMOVED_CODECVT`` macro has been added to make
   the declarations in ``<codecvt>`` available.
 
+- The ``_LIBCPP_ENABLE_CXX26_REMOVED_STRING_RESERVE`` macro has been added to make
+  the function ``std::basic_string<...>::reserve()`` available.
+
 
 Deprecations and Removals
 -------------------------

libcxx/docs/Status/Cxx2cPapers.csv

@@ -37,7 +37,7 @@
 "`P2447R6 <https://wg21.link/P2447R6>`__","LWG","``std::span`` over an initializer list","Kona November 2023","","",""
 "`P2821R5 <https://wg21.link/P2821R5>`__","LWG","``span.at()``","Kona November 2023","","",""
 "`P2868R3 <https://wg21.link/P2868R3>`__","LWG","Remove Deprecated ``std::allocator`` Typedef From C++26","Kona November 2023","","",""
-"`P2870R3 <https://wg21.link/P2870R3>`__","LWG","Remove ``basic_string::reserve()`` From C++26","Kona November 2023","","",""
+"`P2870R3 <https://wg21.link/P2870R3>`__","LWG","Remove ``basic_string::reserve()`` From C++26","Kona November 2023","|Complete|","18.0",""
 "`P2871R3 <https://wg21.link/P2871R3>`__","LWG","Remove Deprecated Unicode Conversion Facets from C++26","Kona November 2023","|Complete|","18.0",""
 "`P2819R2 <https://wg21.link/P2819R2>`__","LWG","Add tuple protocol to complex","Kona November 2023","","",""
 "`P2937R0 <https://wg21.link/P2937R0>`__","LWG","Freestanding: Remove ``strtok``","Kona November 2023","","",""

libcxx/docs/UsingLibcxx.rst

@@ -335,6 +335,9 @@ C++26 Specific Configuration Macros
 **_LIBCPP_ENABLE_CXX26_REMOVED_CODECVT**:
   This macro is used to re-enable all named declarations in ``<codecvt>``.
 
+**_LIBCPP_ENABLE_CXX26_REMOVED_STRING_RESERVE**
+  This macro is used to re-enable the function
+  ``std::basic_string<...>::reserve()``.
 
 Libc++ Extensions
 =================

libcxx/include/string

@@ -173,7 +173,7 @@ public:
     constexpr void resize_and_overwrite(size_type n, Operation op); // since C++23
 
     void reserve(size_type res_arg);                                                            // constexpr since C++20
-    void reserve(); // deprecated in C++20
+    void reserve();                                                                             // deprecated in C++20, removed in C++26
     void shrink_to_fit();                                                                       // constexpr since C++20
     void clear() noexcept;                                                                      // constexpr since C++20
     bool empty() const noexcept;                                                                // constexpr since C++20
@@ -1187,7 +1187,9 @@ public:
 
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __resize_default_init(size_type __n);
 
+#if _LIBCPP_STD_VER < 26 || defined(_LIBCPP_ENABLE_CXX26_REMOVED_STRING_RESERVE)
     _LIBCPP_DEPRECATED_IN_CXX20 _LIBCPP_HIDE_FROM_ABI void reserve() _NOEXCEPT { shrink_to_fit(); }
+#endif
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void shrink_to_fit() _NOEXCEPT;
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void clear() _NOEXCEPT;
 

libcxx/test/libcxx/strings/basic.string/string.capacity/PR53170.pass.cpp

@@ -11,7 +11,7 @@
 // void reserve(); // Deprecated in C++20.
 // void reserve(size_type);
 
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS -D_LIBCPP_ENABLE_CXX26_REMOVED_STRING_RESERVE
 
 // This test ensures that libc++ implements https://wg21.link/P0966R1 (reserve never shrinks)
 // even before C++20. This is required in order to avoid ODR violations because basic_string::reserve(size)

libcxx/test/std/strings/basic.string/string.capacity/reserve.deprecated_in_cxx20.verify.cpp

@@ -10,7 +10,7 @@
 
 // void reserve(); // Deprecated in C++20
 
-// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++26
 
 #include <string>
 

libcxx/test/std/strings/basic.string/string.capacity/reserve.pass.cpp

@@ -8,9 +8,9 @@
 
 // <string>
 
-// void reserve(); // Deprecated in C++20.
+// void reserve(); // Deprecated in C++20, removed in C++26.
 
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS -D_LIBCPP_ENABLE_CXX26_REMOVED_STRING_RESERVE
 
 #include <string>
 #include <stdexcept>

libcxx/test/std/strings/basic.string/string.capacity/reserve.removed_in_cxx26.verify.cpp

@@ -0,0 +1,20 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <string>
+
+// void reserve(); // Removed in C++26
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20, c++23
+
+#include <string>
+
+void f() {
+  std::string s;
+  s.reserve(); // expected-error {{too few arguments to function call}}
+}