[libc++] Make sure basic_string::reserve(n) never shrinks in all Standard modes
Since basic_string::reserve(n) is instantiated in the shared library but also
available to the compiler for inlining, its definition should not depend on
things like the Standard mode in use. Indeed, that flag may not match between
how the shared library is compiled and how users are compiling their own code,
resulting in ODR violations.
However, note that we retain the behavior of basic_string::reserve() to
shrink the string for backwards compatibility reasons. While it would
technically be conforming to not shrink, we believe user expectation is
for it to shrink, and so existing code might have been written based on
that assumption. We prefer to not break such code, even though that makes
basic_string::reserve() and basic_string::reserve(0) not equivalent anymore.
Fixes llvm-project#53170
Differential Revision: https://reviews.llvm.org/D117332
NOKEYCHECK=True
GitOrigin-RevId: 0407ab4114dbdbd1845df712639bdbc84ec6df2c
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index b35c8a3..7210e19 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -115,6 +115,18 @@
You must now explicitly initialize with a ``chrono::month`` and
``chrono::weekday_indexed`` instead of "meh, whenever".
+- C++20 requires that ``std::basic_string::reserve(n)`` never reduce the capacity
+ of the string. (For that, use ``shrink_to_fit()``.) Prior to this release, libc++'s
+ ``std::basic_string::reserve(n)`` could reduce capacity in C++17 and before, but
+ not in C++20 and later. This caused ODR violations when mixing code compiled under
+ different Standard modes. After this change, libc++'s ``std::basic_string::reserve(n)``
+ never reduces capacity, even in C++17 and before.
+ C++20 deprecates the zero-argument overload of ``std::basic_string::reserve()``,
+ but specifically permits it to reduce capacity. To avoid breaking existing code
+ assuming that ``std::basic_string::reserve()`` will shrink, libc++ maintains
+ the behavior to shrink, even though that makes ``std::basic_string::reserve()`` not
+ a synonym for ``std::basic_string::reserve(0)`` in any Standard mode anymore.
+
ABI Changes
-----------