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relates to:

• SR-6993 Swift imports @optional ObjC methods from superclass protocol as if implemented

Issue Description:

If you have

• a Swift subclass of an ObjC class

• where the ObjC class conforms to an ObjC protocol,

• the protocol declares an init method marked as unavailable,

• the ObjC class (correctly) does not implement the unavailable init method,

• and the Swift subclass declares its own init method

... the Swift compiler will incorrectly require that the unavailable init method be implemented in the Swift subclass.

This does not appear to be an issue for a Swift class conforming to the protocol directly, or a Swift subclass of a Swift class that conforms.

Example:

@protocol SomeObjcProtocol <NSObject>

- (instancetype)initWithFoo:(BOOL)foo __attribute__((unavailable("do not use initWithFoo")));

@end

@interface SomeObjcClass : NSObject <SomeObjcProtocol>

@end

with an empty implementation of SomeObjcClass, we see

class SomeSwiftSubclass: SomeObjcClass {

    init(bar: Bool) {
        super.init()
    }

    // Error: "'required' initializer 'init(foo:)' must be provided by subclass of 'SomeObjcClass'"
}

class AnotherSwiftClass: NSObject, SomeObjcProtocol {

    init(bar: Bool) {
        super.init()
    }

    // No error
}

class AnotherSwiftSubclass: AnotherSwiftClass {
    
    init(baz: Int) {
        super.init(bar: true)
    }

    // No error
}

Inserting the fixit of

    required init(foo: Bool) {
        fatalError("init(foo:) has not been implemented")
    }

allows SomeSwiftSubclass to build. (In fact, it builds without a warning, but that's a separate issue: [SR-2317])