Enable -Wcompat=error in the testsuite

Enabling -Werror=compat in the testsuite allows us to easily see the
impact that a new warning has on code. It also means that in the period
between adding the warning and making the actual breaking change, all
new test cases that are being added to the testsuite will be
forwards-compatible. This is good because it will make the actual
breaking change contain less irrelevant testsuite updates.

Things that -Wcompat warns about are things that are going to break in
the future, so we can be proactive and keep our testsuite
forwards-compatible.

This patch consists of two main changes:

* Add `TEST_HC_OPTS += -Werror=compat` to the testsuite configuration.
* Fix all broken test cases.

Test Plan: Validate

Reviewers: hvr, goldfire, bgamari, simonpj, RyanGlScott

Reviewed By: goldfire, RyanGlScott

Subscribers: rwbarton, carter

GHC Trac Issues: #15278

Differential Revision: https://phabricator.haskell.org/D5200

10 files changed, 34 insertions, 16 deletions

diff --git a/testsuite/tests/gadt/Gadt23_AST.hs b/testsuite/tests/gadt/Gadt23_AST.hs
index 75b07c7611..f1014a6a88 100644
--- a/testsuite/tests/gadt/Gadt23_AST.hs
+++ b/testsuite/tests/gadt/Gadt23_AST.hs
@@ -2,9 +2,11 @@
 
 module Gadt23_AST where
 
+import Data.Kind (Type)
+
 data Exp_;
 
-data AST :: * -> * -> * where
+data AST :: Type -> Type -> Type where
   Var   :: String -> AST Exp_ tag
   Tag   :: tag    -> AST a tag -> AST a tag
 
diff --git a/testsuite/tests/gadt/Session.hs b/testsuite/tests/gadt/Session.hs
index 4403b6f869..2a9b3da6ef 100644
--- a/testsuite/tests/gadt/Session.hs
+++ b/testsuite/tests/gadt/Session.hs
@@ -4,6 +4,8 @@
 
 module Main where
 
+import Data.Kind (Type)
+
 data Zero = Zero
             deriving (Show)
 
@@ -16,7 +18,7 @@ class TyNum a where
 instance TyNum Zero where
 instance (TyNum p) => TyNum (Succ p) where
 
-data List :: * -> * -> * where
+data List :: Type -> Type -> Type where
              Nil :: List a Zero
              Cons :: (TyNum p) => a -> List a p -> List a (Succ p)
 
diff --git a/testsuite/tests/gadt/T14320.hs b/testsuite/tests/gadt/T14320.hs
index 4acd4c8f63..77c68ee92e 100644
--- a/testsuite/tests/gadt/T14320.hs
+++ b/testsuite/tests/gadt/T14320.hs
@@ -2,14 +2,16 @@
 module T14320
 where
 
-data Exp :: * where
+import Data.Kind (Type)
+
+data Exp :: Type where
   Lit  :: (Int -> Exp)
 
-newtype TypedExp :: * -> * where
+newtype TypedExp :: Type -> Type where
   TEGood ::  forall a . (Exp -> (TypedExp a))
 
 -- The only difference here is that the type is wrapped in parentheses,
 -- but GHC 8.0.1 rejects this program
 --
-newtype TypedExpToo :: * -> * where
+newtype TypedExpToo :: Type -> Type where
   TEBad  :: (forall a . (Exp -> (TypedExpToo a)))
diff --git a/testsuite/tests/gadt/T9380.hs b/testsuite/tests/gadt/T9380.hs
index 99dfab8477..9a2d0f7354 100644
--- a/testsuite/tests/gadt/T9380.hs
+++ b/testsuite/tests/gadt/T9380.hs
@@ -4,6 +4,7 @@
 {-# LANGUAGE GADTs #-}
 module Main where
 
+import Data.Kind (Type)
 import Foreign
 import Unsafe.Coerce
 
@@ -13,11 +14,11 @@ newtype S (a :: M) = S Int
 
 data SomeS = forall a . SomeS (S a)
 
-data V0 :: M -> * where
+data V0 :: M -> Type where
   V0A :: Int -> V0 A
   V0B :: Double -> V0 B
 
-data V1 :: M -> * where
+data V1 :: M -> Type where
   V1A :: Int -> V1 A
   V1B :: Double -> V1 B
   V1a :: () -> V1 a
@@ -65,4 +66,4 @@ test2 =
 main = do
   test0 -- no output at all
   test1 -- A
-  test2 -- O_o
\ No newline at end of file
+  test2 -- O_o
diff --git a/testsuite/tests/gadt/gadt20.hs b/testsuite/tests/gadt/gadt20.hs
index c754831ce5..f2f4503c31 100644
--- a/testsuite/tests/gadt/gadt20.hs
+++ b/testsuite/tests/gadt/gadt20.hs
@@ -6,10 +6,12 @@
 
 module Foo where
 
-data Pair :: (* -> *) -> * where
+import Data.Kind (Type)
+
+data Pair :: (Type -> Type) -> Type where
     Pair :: a b -> b -> Pair a
 
-data Sel :: * -> * where
+data Sel :: Type -> Type where
     A :: Sel Bool
     B :: Sel Integer
 
diff --git a/testsuite/tests/gadt/gadt22.hs b/testsuite/tests/gadt/gadt22.hs
index 67d17c7a67..2bd6b74e44 100644
--- a/testsuite/tests/gadt/gadt22.hs
+++ b/testsuite/tests/gadt/gadt22.hs
@@ -8,6 +8,7 @@
 
 module Expr where
 
+import qualified Data.Kind as K (Type)
 import Data.Set (Set)
 
 data Type a where
@@ -15,7 +16,7 @@ data Type a where
     TypeSet     :: Ord a => Type a -> Type (Set a)
     TypeFun     :: Type a -> Type b -> Type (a -> b)
 
-data Expr :: * -> * where
+data Expr :: K.Type -> K.Type where
     Const :: Type a -> a -> Expr a
 
 data DynExpr = forall a. DynExpr (Expr a)
diff --git a/testsuite/tests/gadt/gadt8.hs b/testsuite/tests/gadt/gadt8.hs
index ebd8b04c49..ba5ecaa603 100644
--- a/testsuite/tests/gadt/gadt8.hs
+++ b/testsuite/tests/gadt/gadt8.hs
@@ -6,7 +6,9 @@
 --      kind signatures
 module ShouldCompile where
 
-data Expr :: * -> * where {
+import Data.Kind (Type)
+
+data Expr :: Type -> Type where {
    EInt    :: Int                                 -> Expr Int  ;
    EBool   :: Bool                                -> Expr Bool ;
    EIf     :: (Expr Bool) -> (Expr a) -> (Expr a) -> Expr a    ;
diff --git a/testsuite/tests/gadt/josef.hs b/testsuite/tests/gadt/josef.hs
index 34bd41ba3f..818a6dd20d 100644
--- a/testsuite/tests/gadt/josef.hs
+++ b/testsuite/tests/gadt/josef.hs
@@ -18,6 +18,8 @@
 
 module Bug where
 
+import Data.Kind (Type)
+
 class Category arr where
   idA  :: arr a a
   comp :: arr a b -> arr b c -> arr a c
@@ -51,7 +53,7 @@ data V prod env t where
   Z :: V prod (prod env t) t
   S :: V prod env t -> V prod (prod env x) t
 
-data Lambda terminal (exp :: * -> * -> *) prod env t where
+data Lambda terminal (exp :: Type -> Type -> Type) prod env t where
     Unit :: Lambda foo exp prod env foo
     Var  :: V prod env t -> Lambda terminal exp prod env t
 {-    Lam  :: Lambda terminal exp prod (prod env a) t
diff --git a/testsuite/tests/gadt/karl1.hs b/testsuite/tests/gadt/karl1.hs
index b671db7cd3..e201a711f6 100644
--- a/testsuite/tests/gadt/karl1.hs
+++ b/testsuite/tests/gadt/karl1.hs
@@ -6,7 +6,9 @@
 
 module Expr1 where
 
-data Expr :: * -> * where       -- Not a GADT at all
+import Data.Kind (Type)
+
+data Expr :: Type -> Type where       -- Not a GADT at all
   Const :: Show a => a -> Expr a
                 -- Note the Show constraint here
   Var   :: Var a -> Expr a
diff --git a/testsuite/tests/gadt/karl2.hs b/testsuite/tests/gadt/karl2.hs
index aa96d689d7..1f46df37ee 100644
--- a/testsuite/tests/gadt/karl2.hs
+++ b/testsuite/tests/gadt/karl2.hs
@@ -5,11 +5,13 @@ module Expr0 where
 -- See Trac #301
 -- This one *does* use GADTs (Fct)
 
-data Expr :: * -> * where
+import Data.Kind (Type)
+
+data Expr :: Type -> Type where
   Const :: Show a => a -> Expr a
   Apply :: Fct a b -> Expr a -> Expr b
 
-data Fct :: * -> * -> * where
+data Fct :: Type -> Type -> Type where
   Succ :: Fct Int Int
   EqZero :: Fct Int Bool
   Add :: Fct Int (Int -> Int)