[GHC] #12473: Ambiguous type var with DefaultSignatures and FunctionalDependencies

[GHC]

#12473: Ambiguous type var with DefaultSignatures and FunctionalDependencies
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        Reporter:  dylex             |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 But we ''do'' call a function with the type given by the generic default
 signature!  In effect the generic-default declaration gives rise to a
 definition
 {{{
 $gdm_bar :: forall a b. (Bar a b, Foo a b) => b -> Int
 $gdm_bar b = foo (get b :: a) b
 }}}
 In an instance declaration where you do not give a declaration for `bar`,
 we fill in with this generic default method.  That is
 {{{
 instance blah => Bar t1 t2 where {}
 }}}
 becomes
 {{{
 instance blah => Bar t1 t2 where
   bar = $gdm_bar
 }}}
 If the type of `$gdm_bar` was ambiguous, this really would be a problem.

 But in this case it isn't!  I'll investigate

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12473#comment:2>
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