Specifies that an instance of the type can be default constructed.
u, an arbitrary identifier
The following expressions must be valid and have their specified effects
For objects of non-aggregate class type, a public default constructor must be defined (either user-defined or implicitly defined) to satisfy
Non-const objects of non-class object type are always
Const non-class types are not
Const aggregate types are not
DefaultConstructible if any of their members is an object of non-class type.
Non-object types (function types, reference types, and the (possibly cv-qualified) type void) as well as the const non-object types are never