[GHC] #12907: ExtendedDefaultRules-related regression in GHC 8.0.2

[GHC]

#12907: ExtendedDefaultRules-related regression in GHC 8.0.2
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.0.2
          Component:  Compiler       |           Version:  8.0.2-rc1
  (Type checker)                     |
           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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 Commit
 https://git.haskell.org/ghc.git/commit/9a34bf1985035858ece043bf38b47b6ff4b88efb
 introduced a regression between GHC 8.0.1 and 8.0.2 regarding the behavior
 of `ExtendedDefaultRules`. In GHC 8.0.1, this program

 {{{#!hs
 module Bug where

 default (Bool)
 }}}

 would compile without issue. On GHC 8.0.2, however, it is rejected with
 this error message:

 {{{
 $ /opt/ghc/8.0.2/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:3:1: error:
     • The default type ‘Bool’ is not an instance of ‘Num’
     • When checking the types in a default declaration
 }}}

 As a result, the `shelly` library now fails to compile with GHC 8.0.2 (see
 https://github.com/yesodweb/Shelly.hs/issues/130). A workaround is to
 explicitly enable `ExtendedDefaultRules` at the top of the module.

 Richard, do you understand what's going on here? I'm not sure if this is a
 real regression or just GHC being more particular (admittedly, I don't
 understand many of the intricacies of `ExtendedDefaultRules`).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12907>
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