Add `Matchable` trait

[mbloms]

@smarter

This makes cross-compilation between 2.13 and 3.1 difficult, I guess one can always use @unchecked instead, but then it’s worth asking whether we really need this new trait or we could instead warn by default on downcasting (or at least downcasting from Any).

I think this will always be true of restricting the languages top type. In the short view, the same thing could be achieved by warnings, a weaker top type on the other hand is a long term solution.

A Matchable trait also seems less like a hack than @unchecked warnings to me.

@smarter

The advantage of Matchable is that it prevents us from passing an opaque type, but this only works when the opaque type is defined as opaque type Foo = X , it doesn’t work if it’s defined as opaque type Foo <: SomeTrait = X , which will be upper-bounded by Matchable meaning that downcasting type tests would still be unsound.

I wrote about this before here:

@mbloms

Not allowing downcasting to an opaque type also fits quite nicely with the fact that usually if I define an

opaque type MySpecialBox <: Box = Box

I probably don’t want someone to downcast from any Box to MySpecialBox . If on the other hand I have:

opaque type Boxish <: AnyRef = Box

I really see no way to prevent someone from turning something Boxish into a Box . A Top type doesn’t help at all.

Disallowing (via compiler warnings or errors) downcasting to an opaque type makes a lot of sense regardless what it’s upper bound is. This is already the case as of PR #10664.

The issue is the other direction: Someone using something Boxish as a Box or a Foo as an X.
I don’t think there’s a good solution to this, thought I’d be happy if someone proved me wrong. It would be a problem regardless, and it’s good enough that you’re “safe” as long as you don’t have a subtype of Matchable as the upper bound on the opaque type.