is Swift strictly enforcing Class in protocol vars?

Question

A protocol P requires a variable v of class C.

Class X implements protocol P declaring a variable v of class C1 where C1 extends C.

Here's the code:

import Foundation    

class C { }
class C1: C { }

protocol P {
    var v: C { get set }
}

class X: P {
    var v: C1

    init(withV v: C1) {
        self.v = v
    }
}

Xcode complaints with this error:

Type 'X' does not conform to protocol 'P'
Protocol requires property 'v' with type 'C'
Candidate has non-matching type 'C1'

Why is the compiler forcing me to match exactly the same type declared in the protocol?

Edit:

The exact same implementation in Obj-C compiles without errors or warnings

@interface C: NSObject
@end
@implementation C
@end

@interface C1: C
@end
@implementation C1
@end

@protocol P <NSObject>
    @property (nonatomic, strong) C *v;
@end

@interface X: NSObject <P>
    @property (nonatomic, strong) C1 *v;
@end
@implementation X
@end

Answer 1

It isn't forcing you to match exactly, but it's forcing you to avoid runtime issues. Your setup can mean that you pass a reference to something claiming to conform to P but which doesn't actually conform. This is because P says that you can set v to any C, but X says you can set v to any C1.

So, if you were to create another subclass of C, C2, and try to use that, everything would fall apart.

I'd expect your obj-c example to complain that you haven't implemented or synthesised the property specified to exist in the protocol...