[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
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 I like Richard's visible type pattern suggestion far more than trying to
 fiddle with `InstanceSigs`. (Well, except for the fact that we don't
 currently have visible type patterns. But I'll try not to be impatient.)

 If we do go down this route, I suppose we'd need to eta-expand //all//
 arguments of a default function when desugaring `DefaultSignatures` so
 that something like this would work:

 {{{#!hs
 class A t where
   f :: forall m. t m -> forall x. Monoid x -> m

 instance A [] where
   f @m tm @x = df @[] @m tm @x
 }}}

 Since I believe we'd need to visibly apply `@x` there in order to avoid
 being ensnared in ambiguity.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14266#comment:17>
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