[GHC] #16314: Improve confusing error message with MINIMAL pragma

[GHC]

#16314: Improve confusing error message with MINIMAL pragma
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        Reporter:  lerkok            |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.3
      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 lerkok):

 Another observation. I've just found out that if you try this:

 {{{#!hs
 class X a where
   foo :: a

   {-# MINIMAL #-}
 }}}

 Then GHC says:

 {{{
 a.hs:1:1: warning:
     • The MINIMAL pragma does not require:
         ‘foo’
       but there is no default implementation.
     • In the class declaration for ‘X’
 }}}

 which is absolutely fantastic! So, it does implement the other side of the
 check. What's missing is if `MINIMAL` requires something, yet you also
 give a default definition for it, then it keeps quiet about it.

 So this request seems to fall well within the current practice of warning
 about these cases.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16314#comment:5>
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