List rules for `Coercible` instances

:info on Data.Coerce.Coercible doesn't display any instances, it's a special class so it should be special-cased

ghci> :i Coercible 
type role Coercible representational representational
class a ~R# b => Coercible (a :: k) (b :: k)
        -- Defined in ‘GHC.Types’

The base case would be easy to display:

instance Coercible a a -- Generated on the fly

but it's trickier to describe the … instance succinctly:

instance Coercible b b' => Coercible (D a b c) (D a b' c')

same for newtypes.

---

Maybe it's better to list concrete examples

coerce @(Reader _ _) 
  :: Reader a b -> (a -> Identity b)

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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