COMPLETE sets aren't read from external packages

If you define these two modules:

{-# 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 #-}

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:

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.)

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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CC	mpickering, rwbarton
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