[GHC] #13321: Importing a bundled pattern "infects" all other imports of that pattern

[GHC]

#13321: Importing a bundled pattern "infects" all other imports of that pattern
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        Reporter:  ezyang            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  low               |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by ezyang:

Old description:

> Consider:
>
> {{{
> {-# LANGUAGE PatternSynonyms #-}
> unit p where
>     module A0 where
>         data G = G Int
>     module A1 where
>         import A0
>         pattern X x = G x
>     module A(G(X)) where
>         import A0
>         import A1
>     module B(G) where
>         import A
>     module C(pattern X) where
>         import A
>     module D(N.G, pattern N.X) where
>         import B
>         import C
>         import qualified A0 as N
>         import qualified A1 as N
> }}}
>
> When we look at the interface for D, we see that N is bundled with D.
> This is a little surprising because I explicitly exported the G and X
> from A0 and A1, where no bundling took place.
>
> I suppose the current semantics might also be alright, but we should
> explicitly say so. The spec in the wiki is silent in this respect:
> https://ghc.haskell.org/trac/ghc/wiki/PatternSynonyms/AssociatingSynonyms

New description:

 Consider:

 {{{
 {-# LANGUAGE PatternSynonyms #-}
 unit p where
     module A0 where
         data G = G Int
     module A1 where
         import A0
         pattern X x = G x
     module A(G(X)) where
         import A0
         import A1
     module B(G) where
         import A
     module C(pattern X) where
         import A
     module D(N.G, pattern N.X) where
         import B
         import C
         import qualified A0 as N
         import qualified A1 as N
 }}}

 When we look at the interface for D, we see that X is bundled with G. This
 is a little surprising because I explicitly exported the G and X from A0
 and A1, where no bundling took place.

 I suppose the current semantics might also be alright, but we should
 explicitly say so. The spec in the wiki is silent in this respect:
 https://ghc.haskell.org/trac/ghc/wiki/PatternSynonyms/AssociatingSynonyms

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13321#comment:4>
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