[GHC] #12700: Don't warn about redundant constraints for type equalities

[GHC]

#12700: Don't warn about redundant constraints for type equalities
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        Reporter:  crockeea          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
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 crockeea):

 I'm not sure why the extra syntax is needed, i.e. how is `let b ~ Foo a`
 different from `b ~ Foo a`?
 I'm intending, as hsyl20 indicated, to do the former, but apparently using
 the `b ~ Foo a` implies, and is more general than, `let b ~ Foo a`. Is
 there no way for GHC to turn the signature
 `forall a b . (b ~ Foo a) => a -> b` into `forall a . (let b ~ Foo a) => a
 -> b` or even `forall a . a -> Foo a`?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12700#comment:6>
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