Add decidable equality class

Currently, we have a TestEquality class

class TestEquality f where
  testEquality :: f a -> f b -> Maybe (a :~: b) 

It would be nice to add a class for fully decidable equality. There are a few options, but this one gets to the point rather quickly:

data EqDec a b where
  NotEqual :: (forall c . a :~: b -> c) -> EqDec a b
  Equal :: EqDec a a

class DecEq f where
  decEq :: f a -> f b -> EqDec a b

Trac metadata

Trac field	Value
Version	8.0.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Core Libraries
Test case
Differential revisions
BlockedBy
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Blocking
CC	ekmett
Operating system
Architecture