[GHC] #10990: Checking whether a default declaration is an instance of a defaultable typeclass is broken

[GHC]

#10990: Checking whether a default declaration is an instance of a defaultable
typeclass is broken
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        Reporter:  kanetw            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  low               |            Milestone:
       Component:  Compiler (Type    |              Version:  7.10.2
  checker)                           |
      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 kanetw):

 Sure.
 {{{
 % ./inplace/bin/ghc-stage2 --interactive -dtrace-level1 -ddump-tc-trace
 > default (Maybe Int)
 ...
 Adding error:
   <interactive>:1:1: error:
       No instance for (Num (Maybe Int))
         arising from a 'default' declaration
       When checking the types in a default declaration
 ...
 }}}.

 I might do a fix for this after I finish #10971. It's not a huge problem,
 but if we don't check for default declaration validity (which might be
 useful with a possible #10972 implementation) we should adjust the
 code/document it.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10990#comment:2>
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