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{-# LANGUAGE QuantifiedConstraints #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}
module T17267b where
-- Now rejected
uc :: a -> b
uc = oops where
oops :: (a ~ b => a ~ b) => a -> b
oops x = x
{-
Consider the ambiguity check for oops.
[G] (a ~ b => a ~ b)
[W] (a ~ b => a ~ b)
==>
[G] (a ~ b => a ~ b)
[G] (a ~# b) was [G] (a ~ b) [G] a ~# b
kick out the QC and (old) (a~b)
[G] (b ~ b => b ~ b) Quantified constraint
[G] (a ~# b) was [G] (b ~ b) [G] a ~# b
[W] (a~b) DictCt
Wanted is rewritten
(b~b) DictCt
-}
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