[GHC] #14322: Simplifying an instance context makes a rewrite rule no longer typecheck

[GHC]

#14322: Simplifying an instance context makes a rewrite rule no longer typecheck
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.1
  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):

 Here's a simpler example:
 {{{
 {-# LANGUAGE GADTs, FlexibleContexts, NoMonoLocalBinds #-}

 data T a where
   MkT :: Eq a => a -> T a

 f :: T [b] -> Bool
 f (MkT xs) = g xs

 g :: Eq [b] => [b] -> Bool
 g xs = xs == xs
 }}}
 We get
 {{{
 T14322.hs:12:6: warning: [-Wsimplifiable-class-constraints]
     * The constraint `Eq [b]' matches an instance declaration
       instance Eq a => Eq [a] -- Defined in `GHC.Classes'
       This makes type inference for inner bindings fragile;
         either use MonoLocalBinds, or simplify it using the instance
 }}}
 But you can't simplify `g`'s type signature, becuase then at the call site
 in `f` we get a wanted `Eq a` dictionary which we can't get from a given
 `Eq [a]` dictionary bound by the existential.

 The exact same thing is happening with your RULE.

 I don't see a great solution here.  Personally I's use `MonoLocalBinds` to
 suppress the warning.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14322#comment:1>
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