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

2 files changed, 6 insertions, 5 deletions

diff --git a/testsuite/tests/overloadedrecflds/should_run/overloadedrecflds_generics.hs b/testsuite/tests/overloadedrecflds/should_run/overloadedrecflds_generics.hs
index c2b4bd6120..56f93fb7af 100644
--- a/testsuite/tests/overloadedrecflds/should_run/overloadedrecflds_generics.hs
+++ b/testsuite/tests/overloadedrecflds/should_run/overloadedrecflds_generics.hs
@@ -17,14 +17,15 @@
 import GHC.Generics
 import Data.Data
 import Data.Proxy
+import Data.Kind
 
-type family FirstSelector (f :: * -> *) :: Meta
+type family FirstSelector (f :: Type -> Type) :: Meta
 type instance FirstSelector (M1 D x f) = FirstSelector f
 type instance FirstSelector (M1 C x f) = FirstSelector f
 type instance FirstSelector (a :*: b)  = FirstSelector a
 type instance FirstSelector (M1 S s f) = s
 
-data SelectorProxy (s :: Meta) (f :: * -> *) a = SelectorProxy
+data SelectorProxy (s :: Meta) (f :: Type -> Type) a = SelectorProxy
 type SelectorProxy' (s :: Meta) = SelectorProxy s Proxy ()
 
 -- Extract the first selector name using GHC.Generics
diff --git a/testsuite/tests/overloadedrecflds/should_run/overloadedrecfldsrun07.hs b/testsuite/tests/overloadedrecflds/should_run/overloadedrecfldsrun07.hs
index c492ac87a1..fdd81c213d 100644
--- a/testsuite/tests/overloadedrecflds/should_run/overloadedrecfldsrun07.hs
+++ b/testsuite/tests/overloadedrecflds/should_run/overloadedrecfldsrun07.hs
@@ -1,10 +1,10 @@
 {-# LANGUAGE DataKinds
+           , PolyKinds
            , FlexibleContexts
            , FlexibleInstances
            , GADTs
            , MultiParamTypeClasses
            , OverloadedLabels
-           , TypeInType
            , ScopedTypeVariables
            , TypeApplications
            , TypeOperators
@@ -13,13 +13,13 @@
 
 import GHC.OverloadedLabels
 import GHC.Records
-import GHC.TypeLits hiding (type (*))
+import GHC.TypeLits
 import Data.Kind
 
 data Label (x :: Symbol) = Label
 data Labelled x a = Label x := a
 
-data Rec :: forall k. [(k, *)] -> * where
+data Rec :: forall k. [(k, Type)] -> Type where
   Nil  :: Rec '[]
   (:>) :: Labelled x a -> Rec xs -> Rec ('(x, a) ': xs)
 infixr 5 :>