[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:  new
        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):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Wow, those constraints are really off! To highlight how bad the behavior
 was in 8.0.1, even this typechecks!

 {{{#!hs
 class (MonadLogger m, MonadIO m) => MonadLoggerIO m where
     askLoggerIO :: m (Loc -> LogSource -> LogLevel -> LogStr -> IO ())
     default askLoggerIO :: (Trans.MonadTrans t)
                         => t m (Loc -> LogSource -> LogLevel -> LogStr ->
 IO ())
     askLoggerIO = Trans.lift askLoggerIO
 }}}

 I'll draft up a patch for the 8.0.2 users' guide explaining this
 difference the a similar vein as ead83db8a7db772a9f248af9767a4283218a5c9f.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12784#comment:5>
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