"Occurs check" not considered when reducing closed type families

Hello,

consider the following example:

{-# LANGUAGE TypeFamilies #-}

type family Same a b where
  Same a a = Int
  Same a b = Char

example :: Same a [a] -> a
example = undefined

bad :: a
bad = example 'c'

This program should type check, using the following reasoning: a and [a] can never be equal, therefore the first equation for Same does not apply, and so we can fall trough to the second one, thus reducing Same to Char. Therefore, bad should be accept.

GHC rejects this program with the following error:

   • Couldn't match expected type ‘Same a [a]’ with actual type ‘Char’
    • In the first argument of ‘example’, namely ‘'c'’
      In the expression: example 'c'
      In an equation for ‘bad’: bad = example 'c'
    • Relevant bindings include bad :: a (bound at tmp/test.hs:11:1)

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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