Change GHC.Exts.Any to a type family

I just had this slightly alarming interchange with GHCi:

Prelude> import Data.Type.Equality
Prelude Data.Type.Equality> import GHC.Exts
Prelude Data.Type.Equality GHC.Exts> :kind! ((Any :: Bool) == (Any :: Bool))
((Any :: Bool) == (Any :: Bool)) :: Bool
= 'False

After staring at the result in disbelief, I figured out why. The instance for == at kind Bool looks like this:

type family EqBool a b where
  EqBool False False = True
  EqBool True  True  = True
  EqBool a     b     = False
type instance (a :: Bool) == (b :: Bool) = EqBool a b

Well, Any isn't False, Any isn't True, so Any == Any must be False!

The solution to this, of course, is to make Any a type family, not a datatype. Then, it wouldn't be apart from the equations in EqBool. I believe this idea has been floated previously but was not implemented as it would have disturbed !TypeLits and/or singletons. These libraries have been updated, and it's time.

I'm happy to do this myself in due course.

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Trac field	Value
Version	7.8.2
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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