[GHC] #14547: Wrong warning by -Wincomplete-patterns

[GHC]

#14547: Wrong warning by -Wincomplete-patterns
-------------------------------------+-------------------------------------
        Reporter:  YoshikuniJujo     |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  low               |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  incomplete-
                                     |  patterns OverloadedLists,
                                     |  PatternMatchWarnings TypeFamilies
Operating System:  Linux             |         Architecture:  x86
 Type of failure:  Incorrect         |            Test Case:
  error/warning at compile-time      |  deSugar/should_compile/T14547
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4624
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"361d23a8ebb44f5df5167306d7b98d8bd1724e06/ghc" 361d23a/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="361d23a8ebb44f5df5167306d7b98d8bd1724e06"
 Normalize the element type of ListPat, fix #14547

 The element type of `List` maybe a type family instacen, rather than a
 trivial type.  For example in Trac #14547,

 ```
 {-# LANGUAGE TypeFamilies, OverloadedLists #-}

 class Foo f where
         type It f
         foo :: [It f] -> f

 data List a = Empty | a :! List a deriving Show

 instance Foo (List a) where
         type It (List a) = a
         foo [] = Empty
         foo (x : xs) = x :! foo xs
 ```

 Here the element type of `[]` is `It (List a)`, we should also normalize
 it as `a`.

 Test Plan: make test TEST="T14547"

 Reviewers: bgamari

 Reviewed By: bgamari

 Subscribers: thomie, carter

 GHC Trac Issues: #14547

 Differential Revision: https://phabricator.haskell.org/D4624
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14547#comment:12>
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