[GHC] #11793: Confusing type error from constrained class method

[GHC]

#11793: Confusing type error from constrained class method
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        Reporter:  gridaphobe        |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  8.0.1-rc2
      Resolution:                    |             Keywords:  error-message
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 The original issue was this.  Haskell has always allowed you to write
 {{{
 class C a where
   foo :: forall b. Ord b => blah
 }}}
 (I'm using an explicit forall, which original Haskell didn't, but only to
 stress where the quantification is.)

 But Haskell 98 did not allow
 {{{
 class C a where
   foo :: Ord a => blah
 }}}
 which added an extra constraint on the class variable 'a'.

 This was not checked by GHC 7.10, see #7854.  I added the test in
 `f66e0e6`.

 But if we have implicit params should we allow
 {{{
 class C a where
   foo :: ?x::Int => a -> a
 }}}
 Presumably yes.  So give a constraint with free tyvars `pred_tvs`, the
 test should perhaps be

 * `pred_tvs` is not empty, and
 * All of the `pred_tvs` are class variables

 Under those circumstances, reject, unless `ConstrainedClassMethods` is on.
 I'll do that; it's easy.

 Simon

 GHC has no truck with such restrictions, hence the extension.  But when
 you don't have the extension on

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11793#comment:1>
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