[GHC] #12702: Don't warn about redundant constraints for necessary

[GHC]

#12702: Don't warn about redundant constraints for necessary
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           Reporter:  crockeea       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 In the attached example, GHC warns that the constraint on line 16 of
 Main.hs is redundant.

 {{{
 • Redundant constraint: e ~ FGCD r s
 • In the type signature for:
     foo :: (e ~ FGCD r e', e' ~ (e * (s / r)), e ~ FGCD r s) =>
             Proxy s -> Proxy r -> ()
 }}}

 However, when I remove that constraint, I get the following error:

 {{{
 • Occurs check: cannot construct the infinite type:
     t0 ~ Factored.FMul (FGCD r t0) (Factored.FDiv s r)
   The type variable ‘t0’ is ambiguous
 • In the ambiguity check for ‘foo’
   To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
   In the type signature:
     foo :: forall e e' r s.
            (e ~ FGCD r e', e' ~ (e * (s / r))) => Proxy s -> Proxy r -> ()
 }}}

 I'm not entirely sure if the error is correct or not; I can't figure out
 what it's getting on about.

 So: if the constraint is required to make `foo` compile, then it is most
 certainly ''not'' redundant, and there shouldn't be a warning suggesting
 its removal. If the constraint really isn't required, then the error is a
 bug.

 (I apologize for the second bug involving singletons in as many days; I
 simply don't know an easy way to remove that dependency.)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12702>
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