Kind TyCons: require KindSignatures, not DataKinds

  Uses of a TyCon in a kind signature required users to enable
  DataKinds, which didn't make much sense, e.g. in

    type U = Type
    type MyMaybe (a :: U) = MyNothing | MyJust a

  Now the DataKinds error is restricted to data constructors;
  the use of kind-level type constructors is instead gated behind
  -XKindSignatures.

  This patch also adds a convenience pattern synonym for patching
  on both a TyCon or a TcTyCon stored in a TcTyThing, used in
  tcTyVar and tc_infer_id.

  fixes #20873

7 files changed, 61 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_compile/T20873.hs b/testsuite/tests/typecheck/should_compile/T20873.hs
new file mode 100644
index 0000000000..825b12aece
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T20873.hs
@@ -0,0 +1,23 @@
+
+{-# LANGUAGE GADTSyntax, StandaloneKindSignatures, NoDataKinds #-}
+
+module T20873 where
+
+import Data.Kind ( Type )
+
+type U a = Type
+type V a = a
+
+type MyMaybe1 :: U Type -> U Type
+data MyMaybe1 a = MyJust1 a | MyNothing1
+
+type MyMaybe2 :: V Type -> V Type
+data MyMaybe2 a = MyJust2 a | MyNothing2
+
+data MyMaybe3 (a :: U Type) :: U Type where
+  MyJust3    :: a -> MyMaybe3 a
+  MyNothing3 ::      MyMaybe3 a
+
+data MyMaybe4 (a :: V Type) :: V Type where
+  MyJust4    :: a -> MyMaybe4 a
+  MyNothing4 ::      MyMaybe4 a
diff --git a/testsuite/tests/typecheck/should_compile/T20873b.hs b/testsuite/tests/typecheck/should_compile/T20873b.hs
new file mode 100644
index 0000000000..5abe5942a7
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T20873b.hs
@@ -0,0 +1,11 @@
+
+{-# LANGUAGE NoDataKinds #-}
+
+module T20873b where
+
+import T20873b_aux (P)
+
+type Q = P
+  -- P = 'MkA is a promoted data constructor,
+  -- but we should still allow users to use P with -XNoDataKinds,
+  -- to avoid implementation details of M1 leaking.
diff --git a/testsuite/tests/typecheck/should_compile/T20873b_aux.hs b/testsuite/tests/typecheck/should_compile/T20873b_aux.hs
new file mode 100644
index 0000000000..efd498471a
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T20873b_aux.hs
@@ -0,0 +1,7 @@
+
+{-# LANGUAGE DataKinds #-}
+
+module T20873b_aux where
+
+data A = MkA
+type P = 'MkA
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index a3c89c0ab0..8b31c8b3cc 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -804,3 +804,6 @@ test('T20584b', normal, compile, [''])
 test('T20588b', [extra_files(['T20588b.hs', 'T20588b.hs-boot', 'T20588b_aux.hs'])], multimod_compile, ['T20588b_aux.hs', '-v0'])
 test('T20588d', [extra_files(['T20588d.hs', 'T20588d.hs-boot', 'T20588d_aux.hs'])], multimod_compile, ['T20588d_aux.hs', '-v0'])
 test('T20661', [extra_files(['T20661.hs', 'T20661.hs-boot', 'T20661_aux.hs'])], multimod_compile, ['T20661_aux.hs', '-v0'])
+
+test('T20873', normal, compile, [''])
+test('T20873b', [extra_files(['T20873b_aux.hs'])], multimod_compile, ['T20873b', '-v0'])
diff --git a/testsuite/tests/typecheck/should_fail/T20873c.hs b/testsuite/tests/typecheck/should_fail/T20873c.hs
new file mode 100644
index 0000000000..776c061f13
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T20873c.hs
@@ -0,0 +1,11 @@
+
+{-# LANGUAGE GADTSyntax, NoKindSignatures, NoDataKinds #-}
+
+module T20873c where
+
+import Data.Kind ( Type )
+
+type U a = Type
+
+data Foo :: U Int where
+  MkFoo :: Foo
diff --git a/testsuite/tests/typecheck/should_fail/T20873c.stderr b/testsuite/tests/typecheck/should_fail/T20873c.stderr
new file mode 100644
index 0000000000..972d01c583
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T20873c.stderr
@@ -0,0 +1,5 @@
+
+T20873c.hs:10:1: error:
+    • Illegal kind signature ‘Foo’
+        (Use KindSignatures to allow kind signatures)
+    • In the data declaration for ‘Foo’
diff --git a/testsuite/tests/typecheck/should_fail/all.T b/testsuite/tests/typecheck/should_fail/all.T
index 908edcfe27..ff092df478 100644
--- a/testsuite/tests/typecheck/should_fail/all.T
+++ b/testsuite/tests/typecheck/should_fail/all.T
@@ -628,3 +628,4 @@ test('T20542', normal, compile_fail, [''])
 test('T20588', [extra_files(['T20588.hs', 'T20588.hs-boot', 'T20588_aux.hs'])], multimod_compile_fail, ['T20588_aux.hs', '-v0'])
 test('T20588c', [extra_files(['T20588c.hs', 'T20588c.hs-boot', 'T20588c_aux.hs'])], multimod_compile_fail, ['T20588c_aux.hs', '-v0'])
 test('T20189', normal, compile_fail, [''])
+test('T20873c', normal, compile_fail, [''])