Index: include/string
===================================================================
--- include/string
+++ include/string
@@ -956,8 +956,10 @@
     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 = 0);
+    void reserve(size_type __res_arg);
     _LIBCPP_INLINE_VISIBILITY
+    void reserve() _NOEXCEPT {reserve(0);}
+    _LIBCPP_INLINE_VISIBILITY
     void shrink_to_fit() _NOEXCEPT {reserve();}
     _LIBCPP_INLINE_VISIBILITY
     void clear() _NOEXCEPT;
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
@@ -9,7 +9,9 @@
 
 // <string>
 
-// void reserve(size_type res_arg=0);
+// Split into two calls for C++20
+// void reserve();
+// void reserve(size_type res_arg);
 
 #include <string>
 #include <stdexcept>
@@ -44,6 +46,9 @@
         assert(s == s0);
         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
+#endif
     }
 #ifndef TEST_HAS_NO_EXCEPTIONS
     else
@@ -90,6 +95,7 @@
     test(s, 10);
     test(s, 50);
     test(s, 100);
+    test(s, 1000);
     test(s, S::npos);
     }
     }
@@ -121,6 +127,7 @@
     test(s, 10);
     test(s, 50);
     test(s, 100);
+    test(s, 1000);
     test(s, S::npos);
     }
     }