Assignable

A type is Assignable if it is possible to assign one object of the type to another object of that type.

`T`	is type that is a model of Assignable
`t`	is an object of type `T`
`u`	is an object of type `T` or possibly `const T`

Name	Expression	Return type	Semantics
Assignment	`t = u`	`T&`	`t` is equivalent to `u`