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{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, FlexibleContexts, TypeFamilies #-}
module PushedInAsGivens where
type family F a
bar y = let foo :: (F Int ~ [a]) => a -> Int
foo x = length [x,y]
in (y,foo y)
-- This example demonstrates why we need to push in
-- an unsolved wanted as a given and not a given/solved.
-- [Wanted] F Int ~ [beta]
--- forall a. F Int ~ [a] => a ~ beta
-- We push in the [Wanted] as given, it will interact and solve the implication
-- constraint, and finally we quantify over F Int ~ [beta]. If we push it in as
-- Given/Solved, it will be discarded when we meet the given (F Int ~ [a]) and
-- we will not be able to solve the implication constraint.
-- Oct 14: actually this example is _really_ strange, and doesn't illustrate
-- the real issue in #4935, for which there is a separate test
--
-- The example here requires us to infer a type
-- bar :: F Int ~ [a] => ...
-- which is a strange type to quantify over; better to complain about
-- having no instance for F Int.
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