Index: include/string
===================================================================
--- include/string
+++ include/string
@@ -1257,6 +1257,8 @@
 
     _LIBCPP_INLINE_VISIBILITY bool __invariants() const;
 
+    _LIBCPP_INLINE_VISIBILITY void __clear_and_shrink();
+    
     _LIBCPP_INLINE_VISIBILITY
     bool __is_long() const _NOEXCEPT
         {return bool(__r_.first().__s.__size_ & __short_mask);}
@@ -1426,16 +1428,14 @@
             {
                 if (!__str.__is_long())
                 {
-                    clear();
-                    shrink_to_fit();
+                    __clear_and_shrink();
                     __alloc() = __str.__alloc();
                 }
                 else
                 {
                     allocator_type __a = __str.__alloc();
                     pointer __p = __alloc_traits::allocate(__a, __str.__get_long_cap());
-                    clear();
-                    shrink_to_fit();
+                    __clear_and_shrink();
                     __alloc() = _VSTD::move(__a);
                     __set_long_pointer(__p);
                     __set_long_cap(__str.__get_long_cap());
@@ -2125,8 +2125,7 @@
     _NOEXCEPT_(is_nothrow_move_assignable<allocator_type>::value)
 #endif
 {
-    clear();
-    shrink_to_fit();
+    __clear_and_shrink();
     __r_.first() = __str.__r_.first();
     __move_assign_alloc(__str);
     __str.__zero();
@@ -3579,6 +3578,22 @@
     return true;
 }
 
+// __clear_and_shrink
+
+template<class _CharT, class _Traits, class _Allocator>
+inline _LIBCPP_INLINE_VISIBILITY
+void 
+basic_string<_CharT, _Traits, _Allocator>::__clear_and_shrink()
+{
+    clear();
+    if(__is_long())
+    {
+        __alloc_traits::deallocate(__alloc(), __get_long_pointer(), capacity() + 1);
+        __set_long_cap(0);
+        __set_short_size(0);
+    }
+} 
+
 // operator==
 
 template<class _CharT, class _Traits, class _Allocator>
Index: test/libcxx/strings/basic.string/string.modifiers/clear_and_shrink_db1.pass.cpp
===================================================================
--- test/libcxx/strings/basic.string/string.modifiers/clear_and_shrink_db1.pass.cpp
+++ test/libcxx/strings/basic.string/string.modifiers/clear_and_shrink_db1.pass.cpp
@@ -0,0 +1,48 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <string>
+
+// Call __clear_and_shrink() and ensure string invariants hold
+
+#if _LIBCPP_DEBUG >= 1
+
+#define _LIBCPP_ASSERT(x, m) ((x) ? (void)0 : std::exit(0))
+
+#include <string>
+#include <cassert>
+
+int main()
+{
+    std::string l = "Long string so that allocation definitely, for sure, absolutely happens. Probably.";
+    std::string s = "short";
+
+    assert(l.__invariants());
+    assert(s.__invariants());
+    
+    s.__clear_and_shrink();
+    assert(s.__invariants());
+    assert(s.size() == 0);
+
+    { 
+    std::string::size_type cap = l.capacity();
+    l.__clear_and_shrink();
+    assert(l.__invariants());
+    assert(l.size() == 0);
+    assert(l.capacity() < cap);
+    }
+}
+
+#else
+
+int main()
+{
+}
+
+#endif