[GHC] #14266: AllowAmbiguousTypes doesn't play well with default class methods

[GHC]

#14266: AllowAmbiguousTypes doesn't play well with default class methods
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           Reporter:  chris-martin   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 This example does compile,

 {{{
 {-# LANGUAGE AllowAmbiguousTypes #-}
 {-# LANGUAGE ExplicitForAll #-}

 class A t where
   f :: forall x m. Monoid x => t m -> m
   f = undefined

 instance A [] where
   f = undefined
 }}}

 and it seems that the following really ought to be equivalent to it, since
 all I have done is remove a method definition which is identical to the
 default:

 {{{
 {-# LANGUAGE AllowAmbiguousTypes #-}
 {-# LANGUAGE ExplicitForAll #-}

 class A t where
   f :: forall x m. Monoid x => t m -> m
   f = undefined

 instance A []
 }}}

 But instead GHC 8.0.2 gives an error of "Could not deduce (Monoid x0)" on
 the instance declaration.

 (I've also posed the same question on stackoverflow:
 https://stackoverflow.com/q/46350839/402884.)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14266>
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