[GHC] #14570: Untouchable error arises from type equality, but not equivalent program with fundeps

[GHC]

#14570: Untouchable error arises from type equality, but not equivalent program
with fundeps
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        Reporter:  lexi.lambda       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.2
  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):

 Consider
 {{{
 {-# LANGUAGE MultiParamTypeClasses, FlexibleContexts, GADTs,
 FunctionalDependencies #-}

 class C k a | k -> a

 data T a where
   MkT :: (a ~ Bool) => a -> T a

 data S a where
   MkS :: (C Bool a) => a -> S a

 f1 (MkT x) = x
 f2 (MkS x) = x
 }}}
 Here, `f2` typechecks fine, yielding `f2 :: S a -> a`.  But `f1` is
 rejected with an untouchable-tyvar error.  But, as in your emample, one
 has a fundep and the other has a type equality.

 Reason: fundeps affect only type inference, causing more unifications to
 happen.  They do not carry evidence, and are not reflected in GHC's
 internal language System FC.  Schrijvers et al have
 [https://people.cs.kuleuven.be/~tom.schrijvers/portfolio/haskell2017a.html
 a paper about this in at HS'17].

 I haven't unpicked your example precisely but I wanted to give you a
 simple case in which fundeps and type equalities behave differently, by
 design.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14570#comment:1>
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