[GHC] #11646: Make pattern synonym export type mismatch a warning

[GHC]

#11646: Make pattern synonym export type mismatch a warning
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           Reporter:  dfeuer         |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1-rc2
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 As I understand it, attempting to associate a pattern synonym with a type
 constructor that it "obviously" doesn't match is an error. This is strange
 for two reasons:

 1. An "erroneous" association cannot compromise type safety.

 2. The test is incomplete.

 Therefore, I believe such mismatches should lead to warnings instead of
 errors.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11646>
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