Index: include/string
===================================================================
--- include/string
+++ include/string
@@ -960,7 +960,7 @@
     void resize(size_type __n, value_type __c);
     _LIBCPP_INLINE_VISIBILITY void resize(size_type __n) {resize(__n, value_type());}
 
-    void reserve(size_type __res_arg);
+    _LIBCPP_INLINE_VISIBILITY void reserve(size_type __res_arg);
     _LIBCPP_INLINE_VISIBILITY void __resize_default_init(size_type __n);
 
     _LIBCPP_INLINE_VISIBILITY
@@ -3136,6 +3136,7 @@
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+inline
 void
 basic_string<_CharT, _Traits, _Allocator>::reserve(size_type __res_arg)
 {
@@ -3143,6 +3144,10 @@
         this->__throw_length_error();
     size_type __cap = capacity();
     size_type __sz = size();
+#if _LIBCPP_STD_VER > 17
+    if (__res_arg <= __cap)
+        return;
+#endif
     __res_arg = _VSTD::max(__res_arg, __sz);
     __res_arg = __recommend(__res_arg);
     if (__res_arg != __cap)
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
 }