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duplicates:

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

Issue Description:

Consider the following code:

protocol ProtocolWithMutableMethod {
   mutating func mutateStuff()
}

extension ProtocolWithMutableMethod {
   mutating func mutateStuff() {
   }
}

struct MyClass: ProtocolWithMutableMethod {
}

let object = MyClass()
object.mutateStuff()

The compiler complains “Cannot use mutating member on immutable value: 'object' is a 'let’ constant” even though object is clearly a reference type where everything is mutable.