[GHC] #10959: MINIMAL pragma and default implementations

[GHC]

#10959: MINIMAL pragma and default implementations
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        Reporter:  basvandijk        |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 > surely the intention of the MINIMAL pragma was to warn if we end up with
 the meth = error "..." implementation that the compiler would supply if
 there was no other implementation at all

 Not only that.  Bur also
 {{{
 class Eq a where
   (==), (/=) :: a -> a -> Bool
   (==) a b = not (a /= b)
   (/=) a b = not (a == b)
 }}}
 Here we have code for both `(==)` and `(/=)` (no `error` here), but the
 MINIMAL pragma allows you to say that you should supply a "real"
 implementation for one or the other.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10959#comment:7>
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