[GHC] #13249: Default signature check can be quite onerous

[GHC]

#13249: Default signature check can be quite onerous
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        Reporter:  bgamari           |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12918            |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by bgamari:

@@ -15,1 +15,1 @@
- (t m))
+ (t n))

New description:

 The `bytes` library currently doesn't compile with GHC `master` due to the
 new check of default signatures (7363d5380e600e2ef868a069d5df6857d9e5c17e,
 #12918). Consider this example,
 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE DefaultSignatures #-}
 module Hi where
 import Control.Monad.Trans.Class

 class MonadGet m where
   type Remaining m :: *

   remaining :: m (Remaining m)
   default remaining :: (MonadTrans t, MonadGet n, m ~ t n) => m (Remaining
 (t n))
   remaining = lift remaining
 }}}

 Patching this up requires a fair amount of hand-holding,
 {{{#!hs
   default remaining :: (MonadTrans t, MonadGet n, m ~ t n, Remaining m ~
 Remaining n, Monad n) => m (Remaining m)
 }}}
 Yuck.

 I suppose this is just how the world works, but I thought I'd leave this
 here in case anyone had a clever idea for improvement.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13249#comment:1>
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