[GHC] #12423: Panic with DeriveAnyClass and DefaultSignatures

[GHC]

#12423: Panic with DeriveAnyClass and DefaultSignatures
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        Reporter:  knrafto           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      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 dfeuer):

 `DefaultSignatures` isn't actually relevant here; `DeriveAnyClass` seems
 to be solely responsible. I get the same error using

 {{{#!hs
 {-# LANGUAGE DeriveAnyClass #-}

 class Eq1 f where
   (==#) :: Eq a => f a -> f a -> Bool

 data Foo a = Foo (Either a a) deriving (Eq, Eq1)
 }}}

 Most surprisingly, to me, these examples compile successfully if `Foo` is
 modified just a drop. Both of the following work:

 {{{#!hs
 data Foo2 a = Foo2 a deriving (Eq, Eq1)
 data Foo3 a b = Foo3 (Either a b) deriving (Eq, Eq1)
 }}}

 (the latter needing the obvious `Eq a => Eq1 (Either a)` instance.)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12423#comment:1>
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