-Wredundant-constraints emits warning even if constraint is narrowing type

I am not sure if this is the same as #12700 , but when having a constraint that uses type equality to narrow down a type, -Wredundant-constraints still emits the warning. My particular example is:

class RowCons (label :: Symbol) (ty :: k) (tail :: Row k) (row :: Row k)
        | label row -> ty tail, label ty tail -> row

instance (RowDelete s r ~ tail, RowPrepend s ty tail ~ r) => RowCons s ty tail r


get :: forall s ty t r. RowCons s ty t r => Record r -> Proxy s -> ty
get _ _ = undefined


test_rec = get (Proxy :: Record ('Cons "foo" '[String])) (Proxy :: Proxy "foo")

With the RowCons constraint, the inferred type of test_rec is String (or [Char]), but without it it is an ambiguous ty.

If the implementation of the type families is needed, they can be found here: https://github.com/jvanbruegge/Megarecord/blob/f65fa5f29dc393e32d42fc87872416ec02405784/src/Megarecord.hs

I tested this with 8.6.1 and 8.4.3

changed weight to 5

added Tbug Trac import labels

changed the description

mentioned in issue #16154

changed title from -Wredundant-constrains emits warning even if constraint is narrowing type to -Wredundant-constraints emits warning even if constraint is narrowing type

I have not dug into this, but I bet it's the same as #16154, which has a good explanation of the cause.

Trac metadata

Trac field	Value
Related	- → #16154

Trac metadata

Trac field	Value
Component	Compiler → Compiler (Type checker)

added error messages label

added Pnormal label