To be clear, in the general case, this isn't even how the language works—it's just how logic works.
For example, every type that conforms to FixedWidthInteger is a fixed-width integer. Meanwhile, any FixedWidthInteger must be able to hold a value of any conforming type (for example, an Int32 or an Int64). This means that any FixedWidthInteger is not a fixed-width integer. Therefore, it cannot conform to FixedWidthInteger.