Display coaxiom branch incompatibilities in GHCi

Currently the only way to figure out how GHC figured out compatibility of branches in a closed type family is to compile it to .hi and then sieve through --show-iface output.

Something as simple this should suffice:

> :i ==
type family (==) (a :: k) (b :: k) :: Bool
  where
      (==) (f a) (g b) = (f == g) && (a == b)
      (==) a a = 'True    -- incompatible with: 0
      (==) _1 _2 = 'False -- incompatible with: 0, 1
  	-- Defined in ‘Data.Type.Equality’
infix 4 ==

Trac metadata

Trac field	Value
Version	8.4.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
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Display coaxiom branch incompatibilities in GHCi