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Ryan Scott authored
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
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