Mark invalid combinations of anchors as deprecated

[ZevEisenberg]

label.leadingAnchor == view.topAnchor

Expected behavior: a compile-time error, telling you that this combination is invalid.

Actual behavior: a warning telling you that the result of == is unused.

The current behavior is that, in the absence of an override of == for <NSLayoutXAxisAnchor, NSLayoutYAxisAnchor>, it’s falling back to the global == → Equatable conformance for NSObject.

Ideally, we could write the appropriate overrides and mark them as @available(*, unavailable), but unfortunately, Swift is too clever for that. It realizes that there’s no point in choosing that implementation, so it again falls back to the NSObject version.

We can mark the methods as @available(*, deprecated, message: "It’s invalid to mix X and Y axes in this way") and then fatalError or -> Never (or both). But we still get a compile-time warning, not error.

One more sweeping workaround would be to stop using built-in UIKit anchors in favor of custom shorthand anchors, like .leading and .top instead of .leadingAnchor and .topAnchor. Those types can do whatever we want, which in this case would be to not inherit from NSObject, so then we would have more control over which operators are valid. It’s the nuclear option, but it might make layout expressions a little nicer to use.

As a stopgap, we should probably implement and deprecate the invalid combinations, just to improve error messaging.

[ZevEisenberg]

🤔 I wonder what effect this would have on compile time?