[GHC] #11766: Lazy application gives "No instance" error while strict application works

[GHC]

#11766: Lazy application gives "No instance" error while strict application works
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        Reporter:  MichaelK          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1-rc2
      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 simonpj):

 Happily this works in HEAD.  Can someone check on the 8.0 branch?

 It'd might be easier for the regression test to use these defns
 {{{
 t1 = cfun id foo $! Nothing
 t2 = let f = cfun id foo in f Nothing
 t3 = cfun id foo Nothing
 t4 = cfun id foo $ Nothing
 }}}
 But then remove the `Eq a` constraint from the `C` instance, which is
 ambiguous and only resolved by GHCi's generous defaulting.

 Check that `t4` does not work with RC2.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11766#comment:1>
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