duplicates:

• SR-142 mutating function in protocol extension erroneously requires var declaration of class variables

Issue Description:

In a class C<T> where T is constrained to be a reference type and conform to a protocol which is not, Swift does not recognize that T should be treated as a reference type and mutations should be allowed even on {{let}}s of type T.
{{
class ReferenceType {

}

protocol Protocol {
var settable: Bool { get set }
}

class C<T> where T: ReferenceType, T: Protocol {

func f(t: T) {
t.settable = false // cannot assign to property: 't' is a 'let' constant
}

}
}}

Making Protocol inherit from AnyObject or NSObjectProtocol fixes the problem in this case.

In the real-world case that led me to report this, my equivalent of Protocol actually did conform to NSObjectProtocol through UITextInput, but Swift still didn't realize it was a reference type. Only be changing all of the other protocols that Protocol inherited from was I able to convince the compiler that t was a reference type. As t was a commonly used property of C in my case, being forced to make it a var was quite inconvenient. I wasn't able to make a small test case of this, however.