[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):  Phab:D2682
       Wiki Page:                    |
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Comment (by rwbarton):

 I like that your proposal is clear on the status of a program like
 {{{
 class Monad m => MonadSupply m where
   fresh :: m Integer
   default fresh :: (Monad n, MonadTrans t) => t n Integer   -- NB: not m
   fresh = lift fresh
 }}}
 which does seem like a reasonable attempt at writing a correct default
 signature. But it's not obvious how to type check in the presence of what
 amount to two type signatures for the same declaration.

 On the other hand, it's mildly unsatisfactory that in order to write your
 recommended version, you need to enable another language extension that in
 turn enables `MonoLocalBinds` by default, which can affect unrelated
 declarations in the module. It feels like the use of type equalities here
 is fairly mild and shouldn't logically entail the need for reasoning about
 local type equalities in pattern matches and so on.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12784#comment:18>
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