[GHC] #8716: improve AMP join warning

[GHC]

#8716: improve AMP join warning
------------------------------------+-------------------------------------
       Reporter:  aavogt            |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.7
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 Any module that defines a top-level join gets this error:
 {{{
 Local definition of ‛join’ clashes with a future Prelude name - this will
 become an error in GHC 7.10, under the Applicative-Monad Proposal.
 }}}

 This file triggers the warning, but it will keep on working when Prelude
 exports join:
 {{{
 module M1 (M.join) where join x = x
 }}}

 This file will break when Prelude exports join, but there is no warning
 currently given.
 {{{
 module M2 where
 import M1 (join)
 f x = join
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8716>
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