[GHC] #14114: Strange behavior when pattern variables are duplicated on pattern synonym RHS

[GHC]

#14114: Strange behavior when pattern variables are duplicated on pattern synonym
RHS
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           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           Keywords:                 |  Operating System:  Unknown/Multiple
  PatternSynonyms                    |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 These might be two separate bugs, but they're similar enough that I'll
 bundle them under the same ticket.

 If you take a pattern variable bound on the LHS of a unidirectional
 pattern synonym and use it multiple locations on the RHS, then GHC accepts
 it, surprisingly:

 {{{#!hs
 pattern Foo a <- (a,a)
 }}}

 What does `Foo` do? As it turns out, the right-most occurrence of `a` in
 the RHS seems to take precedence:

 {{{
 λ> :i Foo
 pattern Foo :: b -> (a, b)      -- Defined at <interactive>:12:1
 λ> case ('a', 'b') of Foo x -> x
 'b'
 }}}

 I expect the definition of `Foo` to error.

 When duplicate pattern variables are used in the RHS of an implicitly
 bidirectional pattern synonym, then it does error. However, the error
 message is quite misleading:

 {{{
 λ> pattern Foo a = (a,a)

 <interactive>:16:17: error:
     Invalid right-hand side of bidirectional pattern synonym ‘Foo’:
       ‘a’ is not bound by the LHS of the pattern synonym
     RHS pattern: (a, a)
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14114>
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