[GHC] #14237: -Wredundant-constraints incorrectly warns about required type equality constraints in default signatures

[GHC]

#14237: -Wredundant-constraints incorrectly warns about required type equality
constraints in default signatures
-------------------------------------+-------------------------------------
        Reporter:  lexi.lambda       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by lexi.lambda):

 This actually seems worse than I thought—GHC warns even when type
 equalities are clearly required. It seems like perhaps `-Wredundant-
 constraints` in GHC 8.2.1 treats all type equalities as redundant? For
 example, this program produces a warning:

 {{{#!hs
 import GHC.Exts (IsList(..))

 nums :: (Num a, IsList t, Item t ~ a) => t
 nums = fromList [1, 2, 3]
 }}}
 {{{
 /private/tmp/redundant-default-
 constraint/src/RedundantDefaultConstraints.hs:9:1: warning: [-Wredundant-
 constraints]
     • Redundant constraint: Item t ~ a
     • In the type signature for:
            nums :: forall a t. (Num a, IsList t, Item t ~ a) => t
   |
 9 | nums :: (Num a, IsList t, Item t ~ a) => t
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14237#comment:1>
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