Embrace -XTypeInType, add -XStarIsType

Summary:
Implement the "Embrace Type :: Type" GHC proposal,
.../ghc-proposals/blob/master/proposals/0020-no-type-in-type.rst

GHC 8.0 included a major change to GHC's type system: the Type :: Type
axiom. Though casual users were protected from this by hiding its
features behind the -XTypeInType extension, all programs written in GHC
8+ have the axiom behind the scenes. In order to preserve backward
compatibility, various legacy features were left unchanged. For example,
with -XDataKinds but not -XTypeInType, GADTs could not be used in types.
Now these restrictions are lifted and -XTypeInType becomes a redundant
flag that will be eventually deprecated.

* Incorporate the features currently in -XTypeInType into the
  -XPolyKinds and -XDataKinds extensions.
* Introduce a new extension -XStarIsType to control how to parse * in
  code and whether to print it in error messages.

Test Plan: Validate

Reviewers: goldfire, hvr, bgamari, alanz, simonpj

Reviewed By: goldfire, simonpj

Subscribers: rwbarton, thomie, mpickering, carter

GHC Trac Issues: #15195

Differential Revision: https://phabricator.haskell.org/D4748

1 files changed, 7 insertions, 6 deletions

diff --git a/testsuite/tests/simplCore/should_run/T13750a.hs b/testsuite/tests/simplCore/should_run/T13750a.hs
index 7ed72ca241..ac3806e7f9 100644
--- a/testsuite/tests/simplCore/should_run/T13750a.hs
+++ b/testsuite/tests/simplCore/should_run/T13750a.hs
@@ -6,14 +6,15 @@
 {-# LANGUAGE ViewPatterns #-}
 module T13750a where
 
+import Data.Kind (Type)
 import Unsafe.Coerce
 
-type family AnyT :: * where {}
-type family AnyList :: [*] where {}
+type family AnyT :: Type where {}
+type family AnyList :: [Type] where {}
 
-newtype NP (xs :: [*]) = NP [AnyT]
+newtype NP (xs :: [Type]) = NP [AnyT]
 
-data IsNP (xs :: [*]) where
+data IsNP (xs :: [Type]) where
   IsNil  :: IsNP '[]
   IsCons :: x -> NP xs -> IsNP (x ': xs)
 
@@ -34,9 +35,9 @@ pattern x :* xs <- (isNP -> IsCons x xs)
     x :* NP xs = NP (unsafeCoerce x : xs)
 infixr 5 :*
 
-data NS (xs :: [[*]]) = NS !Int (NP AnyList)
+data NS (xs :: [[Type]]) = NS !Int (NP AnyList)
 
-data IsNS (xs :: [[*]]) where
+data IsNS (xs :: [[Type]]) where
   IsZ :: NP x -> IsNS (x ': xs)
   IsS :: NS xs -> IsNS (x ': xs)