[GHC] #11833: GHC can't derive instance of polykinded typeclass for newtype that requires a class constraint

[GHC]

#11833: GHC can't derive instance of polykinded typeclass for newtype that requires
a class constraint
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  RyanGlScott
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #8865, #11837     |  Differential Rev(s):  Phab:D2112
       Wiki Page:                    |
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Comment (by Ryan Scott <ryan.gl.scott@…>):

 In [changeset:"fa86ac7c14b67f27017d795811265c3a9750024b/ghc"
 fa86ac7c/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="fa86ac7c14b67f27017d795811265c3a9750024b"
 Make validDerivPred ignore non-visible arguments to a class type
 constructor

 Summary:
 GHC choked when trying to derive the following:

 ```
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE PolyKinds #-}
 module Example where

 class Category (cat :: k -> k -> *) where
   catId   :: cat a a
   catComp :: cat b c -> cat a b -> cat a c

 newtype T (c :: * -> * -> *) a b = MkT (c a b) deriving Category
 ```

 Unlike in #8865, where we were deriving `Category` for a concrete type
 like
 `Either`, in the above example we are attempting to derive an instance of
 the
 form:

 ```
 instance Category * c => Category (T * c) where ...
 ```

 (using `-fprint-explicit-kinds` syntax). But `validDerivPred` is checking
 if
 `sizePred (Category * c)` equals the number of free type variables in
 `Category * c`. But note that `sizePred` counts both type variables
 //and//
 type constructors, and `*` is a type constructor! So `validDerivPred`
 erroneously rejects the above instance.

 The fix is to make `validDerivPred` ignore non-visible arguments to the
 class
 type constructor (e.g., ignore `*` is `Category * c`) by using
 `filterOutInvisibleTypes`.

 Fixes #11833.

 Test Plan: ./validate

 Reviewers: goldfire, hvr, simonpj, austin, bgamari

 Reviewed By: bgamari

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D2112

 GHC Trac Issues: #11833
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11833#comment:4>
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