1

When I inherit a protocol adding mutating keyword to the property with the same name from the parent protocol, I can't create a concrete type even there's no compile error for the protocols.

Check out this code:

protocol FooProtocol {
    var value: Int { get }
}

protocol MutatingFooProtocol: FooProtocol {
    var value: Int { mutating get }
}

// no compile error so far.

struct Bar: MutatingFooProtocol {

    private var storage: Int
    
    var value: Int {
        mutating get {
            self.storage += 1
            return self.storage
        }
    }
}

This produces compile error that Bar is not conforming to protocol FooProtocol. but adding non mutating var value: Int does not solve the problem. It creates another compile error Invalid redeclaration of 'value'.

2

Not being able to create the conforming type seems correct to me. Any non-mutating getter is a valid mutating getter, but not the other way around. But you're right that there should probably be an error on the definition of MutatingFooProtocol. Mind filing a bug?

3

I totally agree with you that MutatingFooProtocol was not even supposed to be compiled successfully. But I'm not sure this is a bug so that should be fixed or proposed as a evolution.

4

I think it's okay as just a bug. It can probably be a warning to start, so that people's projects don't immediately break.

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5

I filed it SR-7980

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