[GHC] #15052: DeriveAnyClass instances may skip TypeError constraints

[GHC]

#15052: DeriveAnyClass instances may skip TypeError constraints
-------------------------------------+-------------------------------------
        Reporter:  jcpetruzza        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:
                                     |  CustomTypeErrors
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 jcpetruzza):

 Let's forget about the program in comment:1, I understand what GHC is
 doing there and I agree it is the expected behaviour. :)

 The issue is only shown in the program of comment:5. In this program **
 `C` has an empty context**, so writing
 {{{#!haskell
 data TS = TSL | TSR  deriving (Gen.Generic, C)
 }}}

 and writing
 {{{#!haskell
 data TS = TSL | TSR  deriving (Gen.Generic)
 instance C TS
 }}}

 should have the same result, but it doesn't. The first form is accepted,
 the second one is rejected (as expected).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15052#comment:9>
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