-Wredundant-constraints warns about constraints introduced via type synonyms.

I ran into this when using lens, here's a simplied example:

l :: Getter [a] Int
l = to length

In this case, Getter introduces constraints (Functor f, Contravariant f), but the Functor constraint is not used. This leads to a warning from -Wredundant-constraints:

<interactive>:1:6: warning: [-Wredundant-constraints]
    • Redundant constraint: Functor f
    • In the type signature for:
           l :: (Contravariant f, Functor f) => (Int -> f Int) -> [a] -> f [a]

This shouldn't warn, because the constraints were introduced by type synonym and so may well be opaque to the user.

Trac metadata

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