[GHC] #11125: Typechecker can't infer StM m Bool ~ Bool from StM m a ~ a

[GHC]

#11125: Typechecker can't infer StM m Bool ~ Bool from StM m a ~ a
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        Reporter:  nikomi            |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.10.2
  checker)                           |
      Resolution:  invalid           |             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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Comment (by nomeata):

 >  Regardless of the API being confusing or not - using exactly the same
 thing with a vs. any specific type and one works but the other doesn't
 makes me rather confident something is fishy here :-(

 `a` can be instantiated to `StM m a` (note that these are different `a`s),
 but `Bool` cannot be instantiated, and is different from `StM m Bool`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11125#comment:7>
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