Make `AnyClass` generic?

[madsmtm]

Some APIs have bounds on classes, such as -[NSWindow restorationClass]:

@property (nullable, assign) Class<NSWindowRestoration> restorationClass;

This effectively says "whatever class is used here must conform to NSWindowRestoration".

A similar thing is actually needed for -[UIView layerClass], ideally that one should probably say "whatever class is used here must be a subclass of CALayer" (but I don't think Objective-C has a way to express that).

To model this, it might make sense to make AnyClass generic? Something like:

struct AnyClass<Bound = ()> {
    // ...
}

fn restorationClass(&self) -> &'static AnyClass<dyn NSWindowRestoration> { ... }
fn setRestorationClass(&self, r#class: &AnyClass<dyn NSWindowRestoration>) { ... }

fn layerClass(&self) -> &'static AnyClass<CALayer> { ... }

And a few methods for (unsafely probably) converting the class' generic.

[madsmtm]

Then again, there's exactly six APIs in Apple's frameworks that use these bounds:

• -[NSWindow restorationClass].
• -[AVVideoComposition customVideoCompositorClass].
• -[AVMutableVideoComposition customVideoCompositorClass].
• -[NSItemProvider registerObjectOfClass:visibility:loadHandler:].
• -[NSItemProvider canLoadObjectOfClass:].
• -[NSItemProvider loadObjectOfClass:completionHandler:].

So it might also just make sense to instead make these functions generic over T: ClassType + WhateverBound. Or add wrappers that do that.