[GHC] #12151: GHC 8 allows ambiguous type without AllowAmbiguousTypes

[GHC]

#12151: GHC 8 allows ambiguous type without AllowAmbiguousTypes
-------------------------------------+-------------------------------------
           Reporter:  thomie         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #7437
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 This module compiles with GHC 8:

 {{{
 {-# LANGUAGE DefaultSignatures #-}
 -- {-# LANGUAGE AllowAmbiguousTypes #-} -- Not required with GHC 8.0.1.

 module T12151 where

 class Put a where
     put :: a

     default put :: t  -- This should be `a` instead of `t`
     put = undefined
 }}}

 GHC 7.10 required `AllowAmbiguousTypes`, to defer the ambiguity check to
 use sites.

 (Example extracted from the description of #7437)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12151>
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