Index: include/string
===================================================================
--- include/string
+++ include/string
@@ -960,13 +960,17 @@
     void resize(size_type __n, value_type __c);
     _LIBCPP_INLINE_VISIBILITY void resize(size_type __n) {resize(__n, value_type());}
 
+#if !defined(_LIBCPP_BUILDING_LIBRARY)
+    _LIBCPP_INLINE_VISIBILITY
+#endif
     void reserve(size_type __res_arg);
+    _LIBCPP_INLINE_VISIBILITY void __request_capacity(size_type __capacity_arg);
     _LIBCPP_INLINE_VISIBILITY void __resize_default_init(size_type __n);
 
     _LIBCPP_INLINE_VISIBILITY
-    void reserve() _NOEXCEPT {reserve(0);}
+    void reserve() _NOEXCEPT {shrink_to_fit();}
     _LIBCPP_INLINE_VISIBILITY
-    void shrink_to_fit() _NOEXCEPT {reserve();}
+    void shrink_to_fit() _NOEXCEPT {__request_capacity(0);}
     _LIBCPP_INLINE_VISIBILITY
     void clear() _NOEXCEPT;
     _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY
@@ -3136,20 +3140,35 @@
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+#if !defined(_LIBCPP_BUILDING_LIBRARY)
+inline
+#endif
 void
 basic_string<_CharT, _Traits, _Allocator>::reserve(size_type __res_arg)
 {
-    if (__res_arg > max_size())
+#if _LIBCPP_STD_VER > 17
+    size_type __cap = capacity();
+    if (__res_arg <= __cap)
+        return;
+#endif
+    __request_capacity(__res_arg);
+}
+
+template <class _CharT, class _Traits, class _Allocator>
+inline void
+basic_string<_CharT, _Traits, _Allocator>::__request_capacity(size_type __capacity_arg)
+{
+    if (__capacity_arg > max_size())
         this->__throw_length_error();
     size_type __cap = capacity();
     size_type __sz = size();
-    __res_arg = _VSTD::max(__res_arg, __sz);
-    __res_arg = __recommend(__res_arg);
-    if (__res_arg != __cap)
+    __capacity_arg = _VSTD::max(__capacity_arg, __sz);
+    __capacity_arg = __recommend(__capacity_arg);
+    if (__capacity_arg != __cap)
     {
         pointer __new_data, __p;
         bool __was_long, __now_long;
-        if (__res_arg == __min_cap - 1)
+        if (__capacity_arg == __min_cap - 1)
         {
             __was_long = true;
             __now_long = false;
@@ -3158,15 +3177,15 @@
         }
         else
         {
-            if (__res_arg > __cap)
-                __new_data = __alloc_traits::allocate(__alloc(), __res_arg+1);
+            if (__capacity_arg > __cap)
+                __new_data = __alloc_traits::allocate(__alloc(), __capacity_arg+1);
             else
             {
             #ifndef _LIBCPP_NO_EXCEPTIONS
                 try
                 {
             #endif  // _LIBCPP_NO_EXCEPTIONS
-                    __new_data = __alloc_traits::allocate(__alloc(), __res_arg+1);
+                    __new_data = __alloc_traits::allocate(__alloc(), __capacity_arg+1);
             #ifndef _LIBCPP_NO_EXCEPTIONS
                 }
                 catch (...)
@@ -3188,7 +3207,7 @@
             __alloc_traits::deallocate(__alloc(), __p, __cap+1);
         if (__now_long)
         {
-            __set_long_cap(__res_arg+1);
+            __set_long_cap(__capacity_arg+1);
             __set_long_size(__sz);
             __set_long_pointer(__new_data);
         }
Index: test/std/strings/basic.string/string.capacity/reserve.pass.cpp
===================================================================
--- test/std/strings/basic.string/string.capacity/reserve.pass.cpp
+++ test/std/strings/basic.string/string.capacity/reserve.pass.cpp
@@ -30,6 +30,10 @@
     assert(s == s0);
     assert(s.capacity() <= old_cap);
     assert(s.capacity() >= s.size());
+#if TEST_STD_VER > 17
+    // reserve never shrinks as of P0966
+    assert(s.capacity() == old_cap);
+#endif
 }
 
 template <class S>
@@ -46,7 +50,9 @@
         assert(s.capacity() >= res_arg);
         assert(s.capacity() >= s.size());
 #if TEST_STD_VER > 17
-        assert(s.capacity() >= old_cap); // resize never shrinks as of P0966
+        // reserve never shrinks as of P0966
+        if (res_arg <= old_cap)
+            assert(s.capacity() == old_cap);
 #endif
     }
 #ifndef TEST_HAS_NO_EXCEPTIONS
@@ -97,6 +103,24 @@
     test(s, 1000);
     test(s, S::npos);
     }
+    {
+    S s;
+    test(s, 50);
+    test(s, 10);
+    test(s, 5);
+    }
+#if TEST_STD_VER > 17
+    {
+    S s;
+    s.reserve(100);
+    typename S::size_type old_cap = s.capacity();
+    s.reserve(5);
+    // reserve never shrinks as of P0966
+    assert(s.capacity() == old_cap);
+    s.reserve(old_cap - 1);
+    assert(s.capacity() == old_cap);
+    }
+#endif
     }
 #if TEST_STD_VER >= 11
     {
@@ -129,6 +153,24 @@
     test(s, 1000);
     test(s, S::npos);
     }
+    {
+    S s;
+    test(s, 50);
+    test(s, 10);
+    test(s, 5);
+    }
+#if TEST_STD_VER > 17
+    {
+    S s;
+    s.reserve(100);
+    typename S::size_type old_cap = s.capacity();
+    s.reserve(5);
+    // reserve never shrinks as of P0966
+    assert(s.capacity() == old_cap);
+    s.reserve(old_cap - 1);
+    assert(s.capacity() == old_cap);
+    }
+#endif
     }
 #endif
 }