[libc++] Fix wrongly non-inline basic_string::shrink_to_fit

As explained in https://stackoverflow.com/a/70339311/627587, the fact
that shrink_to_fit wasn't defined as inline lead to issues when explicitly
instantiating basic_string. While explicit instantiations are always
somewhat brittle, this one was clearly a bug on our end.

Differential Revision: https://reviews.llvm.org/D115656

(cherry picked from commit bf39e7d)

libcxx/include/string

@@ -3345,6 +3345,7 @@ basic_string<_CharT, _Traits, _Allocator>::reserve(size_type __requested_capacit
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+inline
 void
 basic_string<_CharT, _Traits, _Allocator>::shrink_to_fit() _NOEXCEPT
 {
@@ -3355,6 +3356,7 @@ basic_string<_CharT, _Traits, _Allocator>::shrink_to_fit() _NOEXCEPT
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+inline
 void
 basic_string<_CharT, _Traits, _Allocator>::__shrink_or_extend(size_type __target_capacity)
 {

libcxx/test/std/strings/basic.string/string.capacity/shrink_to_fit.explicit_instantiation.sh.cpp

@@ -0,0 +1,59 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// This test checks that we can explicitly instantiate std::string with a custom
+// character type and traits and then use `shrink_to_fit`. In particular, this is
+// a regression test for the bug that was reported at https://stackoverflow.com/q/69520633/627587
+// and https://seedcentral.apple.com/sm/feedback_collector/radar/85053279.
+
+// RUN: %{cxx} %{flags} %{compile_flags} %s %{link_flags} -DTU1 -c -o %t.tu1.o
+// RUN: %{cxx} %{flags} %{compile_flags} %s %{link_flags} -DTU2 -c -o %t.tu2.o
+// RUN: %{cxx} %{flags} %t.tu1.o %t.tu2.o %{link_flags} -o %t.exe
+
+// UNSUPPORTED: libcpp-has-no-localization
+
+#include <cstdint>
+#include <ios>
+#include <string>
+
+typedef std::uint16_t char16;
+
+struct string16_char_traits {
+  typedef char16 char_type;
+  typedef int int_type;
+
+  typedef std::streamoff off_type;
+  typedef std::mbstate_t state_type;
+  typedef std::fpos<state_type> pos_type;
+
+  static void assign(char_type&, const char_type&) { }
+  static bool eq(const char_type&, const char_type&) { return false; }
+  static bool lt(const char_type&, const char_type&) { return false; }
+  static int compare(const char_type*, const char_type*, size_t) { return 0; }
+  static size_t length(const char_type*) { return 0; }
+  static const char_type* find(const char_type*, size_t, const char_type&) { return nullptr; }
+  static char_type* move(char_type*, const char_type*, size_t) { return nullptr; }
+  static char_type* copy(char_type*, const char_type*, size_t) { return nullptr; }
+  static char_type* assign(char_type*, size_t, char_type) { return nullptr; }
+  static int_type not_eof(const int_type&) { return 0; }
+  static char_type to_char_type(const int_type&) { return char_type(); }
+  static int_type to_int_type(const char_type&) { return int_type(); }
+  static bool eq_int_type(const int_type&, const int_type&) { return false; }
+  static int_type eof() { return int_type(); }
+};
+
+#if defined(TU1)
+template class std::basic_string<char16, string16_char_traits>;
+#else
+extern template class std::basic_string<char16, string16_char_traits>;
+
+int main() {
+    std::basic_string<char16, string16_char_traits> s;
+    s.shrink_to_fit();
+}
+#endif