[GHC] #11835: ApplicativeDo failed to desugar last line with pure $ <expr>

[GHC]

#11835: ApplicativeDo failed to desugar last line with pure $ <expr>
-------------------------------------+-------------------------------------
           Reporter:  Cosmia         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1-rc2
           Keywords:  ApplicativeDo  |  Operating System:  MacOS X
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #11607
Differential Rev(s):                 |         Wiki Page:
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 {{{#!hs
 {-# LANGUAGE ApplicativeDo #-}
 f m = do
   x <- m 1
   y <- m 2
   return $ x + y
 }}}
 f should have type (Applicative f, Num a, Num b) => (a -> f b) -> f b
 but ghc considers f a monad

 maybe similar with #11607

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11835>
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