6.6.1 Types, Only for lvalue

const and volatile are part of an identifier’s type, not its storage class. However, they are often removed from the topmost part of the type when an object’s value is fetched in the evaluation of an expression—exactly at the point when an lvalue becomes an rvalue. These terms arise from the prototypical assignment “L=R”; in which the left side must still refer directly to an object (an lvalue) and the right side need only be a value (an rvalue). Thus, only expressions that are lvalues can be qualified by const or volatile or both.