Functional dependency coverage check does not use type equalities

When checking the coverage condition to validate an instance, we should also consider equality constraints because they have a dependency in both directions.

class C a b | a -> b
instance (a ~ b) => C a b

--  Illegal instance declaration for ‛C a b’
--    The coverage condition fails in class ‛C’
--      for functional dependency: ‛a -> b’
--    Reason: lhs type ‛a’ does not determine rhs type ‛b’
--    Using UndecidableInstances might help
--  In the instance declaration for ‛C a b’

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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CC	iavor.diatchki@gmail.com
Operating system
Architecture