diff --git a/libcxx/docs/ReleaseNotes.rst b/libcxx/docs/ReleaseNotes.rst
--- a/libcxx/docs/ReleaseNotes.rst
+++ b/libcxx/docs/ReleaseNotes.rst
@@ -97,6 +97,12 @@
   from all modes. Their symbols are still provided by the dynamic library for the benefit of
   existing compiled code. All of these functions have always behaved as no-ops.
 
+- Calling ``std::basic_string::reserve()`` used to shrink the string if it could in all Standard modes.
+  Calling ``std::basic_string::reserve(capacity)`` used to shrink the string if it could in C++17 and before,
+  but not in C++20 and later. This behavior was changed so that both these calls never shrink the string
+  consistently in all Standard modes. This is a conforming implementation of the Standard, and this change
+  was done to avoid serious ODR violations when mixing code compiled in different Standard modes.
+
 ABI Changes
 -----------
 
diff --git a/libcxx/include/string b/libcxx/include/string
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -987,7 +987,7 @@
     _LIBCPP_INLINE_VISIBILITY void __resize_default_init(size_type __n);
 
     _LIBCPP_DEPRECATED_IN_CXX20 _LIBCPP_INLINE_VISIBILITY
-    void reserve() _NOEXCEPT {shrink_to_fit();}
+    void reserve() _NOEXCEPT { reserve(0); } // Implementation-specific behavior: we never shrink the string on reserve()
     _LIBCPP_INLINE_VISIBILITY
     void shrink_to_fit() _NOEXCEPT;
     _LIBCPP_INLINE_VISIBILITY
@@ -3266,10 +3266,11 @@
     if (__requested_capacity > max_size())
         this->__throw_length_error();
 
-#if _LIBCPP_STD_VER > 17
-    // Reserve never shrinks as of C++20.
-    if (__requested_capacity <= capacity()) return;
-#endif
+    // Make sure reserve(n) never shrinks. This is technically only required in C++20
+    // and later (since P0966R1), however we provide consistent behavior in all standard
+    // modes because this function is instantiated in the shared library.
+    if (__requested_capacity <= capacity())
+        return;
 
     size_type __target_capacity = _VSTD::max(__requested_capacity, size());
     __target_capacity = __recommend(__target_capacity);
diff --git a/libcxx/test/libcxx/strings/basic.string/string.capacity/PR53170.pass.cpp b/libcxx/test/libcxx/strings/basic.string/string.capacity/PR53170.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/libcxx/strings/basic.string/string.capacity/PR53170.pass.cpp
@@ -0,0 +1,62 @@
+//===----------------------------------------------------------------------===//
+//
+// 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(); // Deprecated in C++20.
+// void reserve(size_type);
+
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
+// 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
+// is compiled into the shared library. Hence, it needs to have the same definition in all standard modes.
+//
+// Reported as https://llvm.org/PR53170.
+
+#include <string>
+#include <stdexcept>
+#include <cassert>
+
+#include "test_macros.h"
+#include "min_allocator.h"
+
+template <class S>
+void test() {
+    // Test that a call to reserve() never shrinks the string.
+    {
+        S s(1000, 'a');
+        typename S::size_type old_cap = s.capacity();
+        s.resize(20);
+        assert(s.capacity() == old_cap);
+
+        s.reserve();
+        assert(s.capacity() == old_cap);
+    }
+
+    // Test that a call to reserve(smaller-than-capacity) never shrinks the string.
+    {
+        S s(1000, 'a');
+        typename S::size_type old_cap = s.capacity();
+        s.resize(20);
+        assert(s.capacity() == old_cap);
+
+        s.reserve(10);
+        assert(s.capacity() == old_cap);
+    }
+}
+
+int main(int, char**) {
+    test<std::string>();
+
+#if TEST_STD_VER >= 11
+    test<std::basic_string<char, std::char_traits<char>, min_allocator<char> > >();
+#endif
+
+    return 0;
+}
diff --git a/libcxx/test/libcxx/strings/basic.string/string.capacity/reserve.pass.cpp b/libcxx/test/libcxx/strings/basic.string/string.capacity/reserve.pass.cpp
deleted file mode 100644
--- a/libcxx/test/libcxx/strings/basic.string/string.capacity/reserve.pass.cpp
+++ /dev/null
@@ -1,50 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// 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(); // Deprecated in C++20.
-
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
-
-#include <string>
-#include <stdexcept>
-#include <cassert>
-
-#include "test_macros.h"
-#include "min_allocator.h"
-
-template <class S>
-void
-test()
-{
-    // Tests that a call to reserve() on a long string is equivalent to shrink_to_fit().
-    S s(1000, 'a');
-    typename S::size_type old_cap = s.capacity();
-    s.resize(20);
-    assert(s.capacity() == old_cap);
-    s.reserve();
-    assert(s.capacity() < old_cap);
-}
-
-int main(int, char**)
-{
-    {
-    typedef std::string S;
-    test<S>();
-    }
-#if TEST_STD_VER >= 11
-    {
-    typedef min_allocator<char> A;
-    typedef std::basic_string<char, std::char_traits<char>, A> S;
-    test<S>();
-    }
-#endif
-
-    return 0;
-}