diff --git a/libcxx/include/string b/libcxx/include/string
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -1549,6 +1549,11 @@
     inline
     void __init(size_type __n, value_type __c);
 
+    // Slow path for the (inlined) copy constructor.
+    // Always externally instantiated and not inlined.
+    // Asserts that __s is zero terminated.
+    void __init_copy_ctor_external(const value_type* __s, size_type __sz);
+
     template <class _InputIterator>
     inline
     _EnableIf
@@ -1840,7 +1845,9 @@
     if (!__str.__is_long())
         __r_.first().__r = __str.__r_.first().__r;
     else
-        __init(_VSTD::__to_address(__str.__get_long_pointer()), __str.__get_long_size());
+        __init_copy_ctor_external(_VSTD::__to_address(__str.__get_long_pointer()),
+                                  __str.__get_long_size());
+
 #if _LIBCPP_DEBUG_LEVEL >= 2
     __get_db()->__insert_c(this);
 #endif
@@ -1854,7 +1861,8 @@
     if (!__str.__is_long())
         __r_.first().__r = __str.__r_.first().__r;
     else
-        __init(_VSTD::__to_address(__str.__get_long_pointer()), __str.__get_long_size());
+        __init_copy_ctor_external(_VSTD::__to_address(__str.__get_long_pointer()),
+                                  __str.__get_long_size());
 #if _LIBCPP_DEBUG_LEVEL >= 2
     __get_db()->__insert_c(this);
 #endif
@@ -1880,6 +1888,25 @@
 #endif
 }
 
+template <class _CharT, class _Traits, class _Allocator>
+void basic_string<_CharT, _Traits, _Allocator>::__init_copy_ctor_external(
+    const value_type* __s, size_type __sz) {
+  pointer __p;
+  if (__sz >= __min_cap) {
+    if (__sz > max_size())
+      this->__throw_length_error();
+    size_t __cap = __recommend(__sz);
+    __p = __alloc_traits::allocate(__alloc(), __cap + 1);
+    __set_long_pointer(__p);
+    __set_long_cap(__cap + 1);
+    __set_long_size(__sz);
+  } else {
+    __p = __get_short_pointer();
+    __set_short_size(__sz);
+  }
+  traits_type::copy(_VSTD::__to_address(__p), __s, __sz + 1);
+}
+
 template <class _CharT, class _Traits, class _Allocator>
 inline
 basic_string<_CharT, _Traits, _Allocator>::basic_string(basic_string&& __str, const allocator_type& __a)
diff --git a/libcxx/test/std/strings/basic.string/string.cons/copy_shrunk_long.pass.cpp b/libcxx/test/std/strings/basic.string/string.cons/copy_shrunk_long.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/strings/basic.string/string.cons/copy_shrunk_long.pass.cpp
@@ -0,0 +1,50 @@
+//===----------------------------------------------------------------------===//
+//
+// 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>
+
+// basic_string(const basic_string<charT,traits,Allocator>& str);
+
+#include <string>
+#include <cassert>
+
+#include "test_macros.h"
+#include "test_allocator.h"
+#include "min_allocator.h"
+
+template <class S>
+void
+test()
+{
+  // Tests that a long string holding a SSO size string results in
+  // an SSO copy constructed value.
+  S s1("1234567890123456789012345678901234567890123456789012345678901234567890");
+  s1.resize(7);
+  S s2(s1);
+  LIBCPP_ASSERT(s2.__invariants());
+  assert(s2 == s1);
+  assert(s2.capacity() < sizeof(S));
+}
+
+int main(int, char**)
+{
+    {
+    typedef test_allocator<char> A;
+    typedef std::basic_string<char, std::char_traits<char>, A> 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;
+}