[GHC] #9972: t7861 gives unexpected result (deferred type error)

9 Jan 2015


      #9972: t7861 gives unexpected result (deferred type error)
-------------------------------------+-------------------------------------
              Reporter:  luite       |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.1-rc1
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
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 I'm not sure why `validate` does not trip on this, but the test case does
 not look obviously disabled to me, so I thought I'd just report it so at
 least it doesn't get unnoticed.

 GHC 7.8:

 {{{
 "Hello"
 t7861: t7861.hs:11:5:
     Couldn't match type ‘a’ with ‘[a]’
       ‘a’ is a rigid type variable bound by
           the type signature for f :: A a -> a at t7861.hs:10:6
     Expected type: A a -> a
       Actual type: A a -> [a]
     Relevant bindings include f :: A a -> a (bound at t7861.hs:11:1)
     In the expression: doA
     In an equation for ‘f’: f = doA
 (deferred type error)
 }}}

 On GHC 7.10 the result is (both with and without optimization)

 {{{
 "Hello"
 "Bad"
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9972
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