[GHC] #14813: EmptyCase thinks pattern match involving type family is not exhaustive, when it actually is

[GHC]

#14813: EmptyCase thinks pattern match involving type family is not exhaustive,
when it actually is
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:
                                     |  PatternMatchWarnings
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 Another example of this phenomenon:

 {{{#!hs
 {-# LANGUAGE EmptyCase #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# OPTIONS_GHC -Wall #-}
 module Bug where

 import Data.Void

 type family F a
 type instance F Int = Void

 fun :: F Int -> a
 fun x = case x of
 }}}

 Compiling this program yields no warnings, as expected. However, if you
 factor out the `Int` part like so:

 {{{#!hs
 fun :: i ~ Int => F i -> a
 }}}

 Then it will yield a warning:

 {{{
 $ /opt/ghc/8.4.3/bin/ghci Bug.hs
 GHCi, version 8.4.3: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:12:9: warning: [-Wincomplete-patterns]
     Pattern match(es) are non-exhaustive
     In a case alternative: Patterns not matched: _ :: F i
    |
 12 | fun x = case x of
    |         ^^^^
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14813#comment:5>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler