[GHC] #12784: Typechecker regression in GHC 8.0.2 involving DefaultSignatures

[GHC]

#12784: Typechecker regression in GHC 8.0.2 involving DefaultSignatures
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        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  merge
        Priority:  highest           |            Milestone:  8.0.2
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             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):  Phab:D2682
       Wiki Page:                    |
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Comment (by ilovezfs):

 Here's another instance in the wild that just got released:

 https://github.com/purescript/purescript/issues/2421.


 {{{
 src/Control/Monad/Supply/Class.hs:30:10: error:
     • Couldn't match type ‘m’ with ‘StateT s m’
       ‘m’ is a rigid type variable bound by
         the instance declaration at
 src/Control/Monad/Supply/Class.hs:30:10
       Expected type: StateT s m Integer
         Actual type: StateT s (StateT s m) Integer
 etc.
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12784#comment:10>
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