[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 simonpj):

 I see

 * `foo` really does have the type `(RealFrac a, Integral b) => a -> b`.
 That is, as GHC claims, the constraint can simply be omitted.

 * But you specifically want to write a less-polymorphic type than the most
 general type.  Another example might be
 {{{
 f :: Ord a => a -> a
 f x = x
 }}}
   The `Ord a` is not needed, but you might want to require it for some
 software engineering reason.  That's your situation here, I believe.

 * It's worth noting that you can certainly say
 {{{
 f :: [a] -> [a]
 f x = x
 }}}
   thus specifying a less polymorphic type than the most general one,
 without causing a warning of any kind.

 This is one of the reasons that `-Wredundant-constraints` is no longer on
 by default, and is not even enabled by `-Wall`.  I'm a bit sad, but the
 annoyance was too great.

 The real solution would be a per-function pragma to say "I intend to have
 a redundant constraint here".

 If I've understood aright, then we need take no action, except add it as
 another use-case for the per-function pragma

 See #11370, #10635

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12700#comment:3>
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