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

9 files changed, 12 insertions, 11 deletions

diff --git a/testsuite/tests/th/T11463.hs b/testsuite/tests/th/T11463.hs
index 1faf5964f4..3cb57d1cea 100644
--- a/testsuite/tests/th/T11463.hs
+++ b/testsuite/tests/th/T11463.hs
@@ -1,5 +1,5 @@
 {-# LANGUAGE TemplateHaskell #-}
-{-# LANGUAGE TypeInType #-}
+{-# LANGUAGE DataKinds, PolyKinds #-}
 module Main where
 
 import Data.Kind
diff --git a/testsuite/tests/th/T11484.hs b/testsuite/tests/th/T11484.hs
index d8c0708bd2..e1e30fc694 100644
--- a/testsuite/tests/th/T11484.hs
+++ b/testsuite/tests/th/T11484.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE TypeInType #-}
+{-# LANGUAGE PolyKinds #-}
 
 module T11484 where
 
diff --git a/testsuite/tests/th/T13642.hs b/testsuite/tests/th/T13642.hs
index 090b891433..ab655c0e4a 100644
--- a/testsuite/tests/th/T13642.hs
+++ b/testsuite/tests/th/T13642.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE GADTs, TypeInType, TemplateHaskell, RankNTypes #-}
+{-# LANGUAGE GADTs, PolyKinds, TemplateHaskell, RankNTypes #-}
 module T13642 where
 
 import Data.Kind (Type)
diff --git a/testsuite/tests/th/T13781.hs b/testsuite/tests/th/T13781.hs
index 7498f56bae..2fd2f1a3d4 100644
--- a/testsuite/tests/th/T13781.hs
+++ b/testsuite/tests/th/T13781.hs
@@ -1,5 +1,5 @@
 {-# LANGUAGE TemplateHaskell #-}
-{-# LANGUAGE TypeInType #-}
+{-# LANGUAGE PolyKinds #-}
 module T13781 where
 
 import Data.Kind
diff --git a/testsuite/tests/th/T14060.hs b/testsuite/tests/th/T14060.hs
index 5527b25b6e..8c4f2ddc95 100644
--- a/testsuite/tests/th/T14060.hs
+++ b/testsuite/tests/th/T14060.hs
@@ -2,7 +2,7 @@
 {-# LANGUAGE ExistentialQuantification #-}
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE TypeFamilyDependencies #-}
-{-# LANGUAGE TypeInType #-}
+{-# LANGUAGE PolyKinds #-}
 module Main where
 
 import Data.Kind
diff --git a/testsuite/tests/th/T14869.hs b/testsuite/tests/th/T14869.hs
index c58d4e2720..4b0dcdc171 100644
--- a/testsuite/tests/th/T14869.hs
+++ b/testsuite/tests/th/T14869.hs
@@ -1,7 +1,7 @@
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE TypeFamilies #-}
-{-# LANGUAGE TypeInType #-}
+{-# LANGUAGE DataKinds #-}
 module T14869 where
 
 import Data.Kind
diff --git a/testsuite/tests/th/T8031.hs b/testsuite/tests/th/T8031.hs
index 9f06c06ed6..08081ed6fd 100644
--- a/testsuite/tests/th/T8031.hs
+++ b/testsuite/tests/th/T8031.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE TemplateHaskell, RankNTypes, TypeOperators, TypeInType,
+{-# LANGUAGE TemplateHaskell, RankNTypes, TypeOperators, DataKinds, PolyKinds,
              GADTs #-}
 
 module T8031 where
@@ -6,7 +6,7 @@ module T8031 where
 import Data.Proxy
 import Data.Kind
 
-data SList :: [k] -> * where
+data SList :: [k] -> Type where
   SCons :: Proxy h -> Proxy t -> SList (h ': t)
 
 $( [d| foo :: forall (a :: k). Proxy a
diff --git a/testsuite/tests/th/TH_RichKinds2.hs b/testsuite/tests/th/TH_RichKinds2.hs
index ab3e107d0f..9ac13a157b 100644
--- a/testsuite/tests/th/TH_RichKinds2.hs
+++ b/testsuite/tests/th/TH_RichKinds2.hs
@@ -8,6 +8,7 @@
 
 module TH_RichKinds2 where
 
+import qualified Data.Kind as K
 import Data.Char
 import Data.List
 import Language.Haskell.TH
@@ -32,7 +33,7 @@ $( let fixKs :: String -> String -- need to remove TH renaming index from k vari
              if length index == 0 then s else
              prefix ++ "0" ++ (fixKs rest)
    in
-   do decls <- [d| data SMaybe :: (k -> *) -> (Maybe k) -> * where
+   do decls <- [d| data SMaybe :: (k -> K.Type) -> (Maybe k) -> K.Type where
                      SNothing :: SMaybe s 'Nothing
                      SJust :: s a -> SMaybe s ('Just a)
 
@@ -42,7 +43,7 @@ $( let fixKs :: String -> String -- need to remove TH renaming index from k vari
       reportWarning (fixKs (pprint decls))
       return decls )
 
-data SBool :: Bool -> * where
+data SBool :: Bool -> K.Type where
   SFalse :: SBool 'False
   STrue :: SBool 'True
 
diff --git a/testsuite/tests/th/TH_RichKinds2.stderr b/testsuite/tests/th/TH_RichKinds2.stderr
index 6b0662218a..8970da8bdb 100644
--- a/testsuite/tests/th/TH_RichKinds2.stderr
+++ b/testsuite/tests/th/TH_RichKinds2.stderr
@@ -1,5 +1,5 @@
 
-TH_RichKinds2.hs:24:4: warning:
+TH_RichKinds2.hs:25:4: warning:
     data SMaybe_0 :: (k_0 -> *) -> GHC.Base.Maybe k_0 -> * where
     SNothing_2 :: SMaybe_0 s_3 'GHC.Base.Nothing
     SJust_4 :: (s_5 a_6) -> SMaybe_0 s_5 ('GHC.Base.Just a_6)