[GHC] #13350: COMPLETE sets aren't read from external packages

28 Feb 2017

      #13350: COMPLETE sets aren't read from external packages
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
  PatternSynonyms                    |
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  error/warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 If you define these two modules:

 {{{#!hs
 {-# LANGUAGE PatternSynonyms #-}
 module Foo where

 data KindRep = KindRepTyConApp
              | KindRepVar
              | KindRepApp
              | KindRepFun
              | KindRepTYPE
              | KindRepTypeLitS
              | KindRepTypeLitD

 pattern KindRepTypeLit :: KindRep
 pattern KindRepTypeLit = KindRepTypeLitD

 {-# COMPLETE KindRepTyConApp, KindRepVar, KindRepApp, KindRepFun,
              KindRepTYPE, KindRepTypeLit #-}
 }}}

 {{{#!hs
 module Bar where

 import Foo

 krInt :: KindRep -> Int
 krInt KindRepTyConApp{} = 0
 krInt KindRepVar{}      = 1
 krInt KindRepApp{}      = 2
 krInt KindRepFun{}      = 3
 krInt KindRepTYPE{}     = 4
 krInt KindRepTypeLit{}  = 5
 }}}

 And you compile `Bar.hs` with `-Wall` on, it will not emit any pattern-
 match exhaustiveness warnings, as expected.

 However, something different happens if you import all of these `KindRep`
 conlikes from `Type.Reflection.Unsafe` instead:

 {{{#!hs
 module Bar where

 import Type.Reflection.Unsafe

 krInt :: KindRep -> Int
 krInt KindRepTyConApp{} = 0
 krInt KindRepVar{}      = 1
 krInt KindRepApp{}      = 2
 krInt KindRepFun{}      = 3
 krInt KindRepTYPE{}     = 4
 krInt KindRepTypeLit{}  = 5
 }}}

 {{{
 $ ~/Software/ghc2/inplace/bin/ghc-stage2 --interactive -Wall Bar.hs
 GHCi, version 8.1.20170228: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bar              ( Bar.hs, interpreted )

 Bar.hs:6:1: warning: [-Wincomplete-patterns]
     Pattern match(es) are non-exhaustive
     In an equation for ‘krInt’:
         Patterns not matched:
             (KindRepTypeLitS _ _)
             (KindRepTypeLitD _ _)
   |
 6 | krInt KindRepTyConApp{} = 0
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^...
 }}}

 When the `COMPLETE` set is defined in a module in an //external package//
 (`base:Type.Reflection.Unsafe`, in this example), GHC doesn't properly
 take it into account when emitting pattern-match exhaustiveness warnings!
 This makes `COMPLETE` sets not terribly useful in practice at the moment.

 (NB: `Type.Reflection.Unsafe`'s definitions of `KindRepTyConApp` //et
 al.// aren't quite the same as what I defined above, but their exact
 definitions aren't important here, just that they have the same names and
 `COMPLETE` set. And this is the only `COMPLETE` set that I could defined
 in the boot libraries at the moment, making it convenient to use.)

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13350
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