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'foo' is expected to type check given that 'bar' and 'qux' type check.

ghc_bug.hs:21:7:
    Couldn't match type `Id (Id Char)' with `Char'
    Expected type: (:.:) Id Id Char
      Actual type: Char
    In the expression: 'x'
    In an equation for `foo': foo = 'x'

{-# LANGUAGE TypeOperators, TypeFamilies #-}
module Main where

type family (:.:) g f a where
 (:.:) g f a = g (f a)
--

type family Id x where
  Id x = x
--

-- Type checks
bar :: Id (Id Char)
bar = 'x'

-- Type checks
qux = 'x' :: (Id :.: Id) Char

-- Couldn't match type `Id (Id Char)' with `Char'
foo :: (Id :.: Id) Char
foo = 'x'

Trac metadata

Trac field	Value
Version	7.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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