Implement P0966 - string::reserve Should Not Shrink

[mclow.lists]

Summary

We already did most of this, so this is a small change.

[ldionne]

Ship it!

[mclow.lists]

Committed as revision 347789

include/string

     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;

test/std/strings/basic.string/string.capacity/reserve.pass.cpp

 
 // <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>

         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

     test(s, 10);
     test(s, 50);
     test(s, 100);
+    test(s, 1000);
     test(s, S::npos);
     }
     }

     test(s, 10);
     test(s, 50);
     test(s, 100);
+    test(s, 1000);
     test(s, S::npos);
     }
     }