From 957fe99f41748b80c3108447e5dd5ce2be962f4e Mon Sep 17 00:00:00 2001
From: Nathan Ward <43621845+NathanSWard@users.noreply.github.com>
Date: Thu, 17 Oct 2019 17:06:27 -0600
Subject: [PATCH] P0966R1 string::reserve() should not shrink (#176)

Fixes #42.
---
 stl/inc/xstring      | 22 ++++++++++++++++++++++
 stl/inc/yvals_core.h | 19 +++++++++++++++++--
 2 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/stl/inc/xstring b/stl/inc/xstring
index 27295e6d40..82d82d73ac 100644
--- a/stl/inc/xstring
+++ b/stl/inc/xstring
@@ -3616,6 +3616,27 @@ public:
         return _Mypair._Myval2._Myres;
     }
 
+#if _HAS_CXX20
+    void reserve(_CRT_GUARDOVERFLOW const size_type _Newcap) { // determine new minimum length of allocated storage
+        if (_Mypair._Myval2._Myres >= _Newcap) { // requested capacity is not larger than current capacity, ignore
+            return; // nothing to do
+        }
+
+        const size_type _Old_size = _Mypair._Myval2._Mysize;
+        _Reallocate_grow_by(
+            _Newcap - _Old_size, [](_Elem* const _New_ptr, const _Elem* const _Old_ptr, const size_type _Old_size) {
+                _Traits::copy(_New_ptr, _Old_ptr, _Old_size + 1);
+            });
+
+        _Mypair._Myval2._Mysize = _Old_size;
+    }
+
+    _CXX20_DEPRECATE_STRING_RESERVE_WITHOUT_ARGUMENT void reserve() {
+        if (_Mypair._Myval2._Mysize == 0 && _Mypair._Myval2._Large_string_engaged()) {
+            _Become_small();
+        }
+    }
+#else // _HAS_CXX20
     void reserve(_CRT_GUARDOVERFLOW const size_type _Newcap = 0) { // determine new minimum length of allocated storage
         if (_Mypair._Myval2._Mysize > _Newcap) { // requested capacity is not large enough for current size, ignore
             return; // nothing to do
@@ -3644,6 +3665,7 @@ public:
 
         // ignore requests to reserve to [_BUF_SIZE, _Myres)
     }
+#endif // _HAS_CXX20
 
     _NODISCARD bool empty() const noexcept {
         return size() == 0;
diff --git a/stl/inc/yvals_core.h b/stl/inc/yvals_core.h
index 1937ed44fa..e8c89e468a 100644
--- a/stl/inc/yvals_core.h
+++ b/stl/inc/yvals_core.h
@@ -44,6 +44,7 @@
 //     (partially implemented)
 // P0898R3 Standard Library Concepts
 // P0919R3 Heterogeneous Lookup For Unordered Containers
+// P0966R1 string::reserve() Should Not Shrink
 // P1227R2 Signed std::ssize(), Unsigned span::size()
 //     (partially implemented)
 // P1357R1 is_bounded_array, is_unbounded_array
@@ -412,7 +413,7 @@
 
 #define _CPPLIB_VER 650
 #define _MSVC_STL_VERSION 142
-#define _MSVC_STL_UPDATE 201909L
+#define _MSVC_STL_UPDATE 201910L
 
 #ifndef _ALLOW_COMPILER_AND_STL_VERSION_MISMATCH
 #ifdef __EDG__
@@ -799,7 +800,21 @@
 #define _DEPRECATE_STDEXT_HASH_UPPER_BOUND
 #endif // ^^^ warning disabled ^^^
 
-// next warning number: STL4024
+// P0966R1 [depr.string.capacity]
+#if _HAS_CXX20 && !defined(_SILENCE_CXX20_STRING_RESERVE_WITHOUT_ARGUMENT_DEPRECATION_WARNING) \
+    && !defined(_SILENCE_ALL_CXX20_DEPRECATION_WARNINGS)
+#define _CXX20_DEPRECATE_STRING_RESERVE_WITHOUT_ARGUMENT                                                             \
+    [[deprecated("warning STL4024: "                                                                                 \
+                 "std::string::reserve() without an argument is deprecated in C++20. "                               \
+                 "To shrink the string's capacity, use std::string::shrink_to_fit() instead. Otherwise, provide an " \
+                 "argument to std::string::reserve(). "                                                              \
+                 "You can define _SILENCE_CXX20_STRING_RESERVE_WITHOUT_ARGUMENT_DEPRECATION_WARNING "                \
+                 "or _SILENCE_ALL_CXX20_DEPRECATION_WARNINGS to acknowledge that you have received this warning.")]]
+#else // ^^^ warning enabled / warning disabled vvv
+#define _CXX20_DEPRECATE_STRING_RESERVE_WITHOUT_ARGUMENT
+#endif // ^^^ warning disabled ^^^
+
+// next warning number: STL4025
 
 
 // LIBRARY FEATURE-TEST MACROS