Fix kind inference for data types. Again.

This patch fixes several aspects of kind inference for data type
declarations, especially data /instance/ declarations

Specifically

1. In kcConDecls/kcConDecl make it clear that the tc_res_kind argument
   is only used in the H98 case; and in that case there is no result
   kind signature; and hence no need for the disgusting splitPiTys in
   kcConDecls (now thankfully gone).

   The GADT case is a bit different to before, and much nicer.
   This is what fixes #18891.

   See Note [kcConDecls: kind-checking data type decls]

2. Do not look at the constructor decls of a data/newtype instance
   in tcDataFamInstanceHeader. See GHC.Tc.TyCl.Instance
   Note [Kind inference for data family instances].  This was a
   new realisation that arose when doing (1)

   This causes a few knock-on effects in the tests suite, because
   we require more information than before in the instance /header/.

   New user-manual material about this in "Kind inference in data type
   declarations" and "Kind inference for data/newtype instance
   declarations".

3. Minor improvement in kcTyClDecl, combining GADT and H98 cases

4. Fix #14111 and #8707 by allowing the header of a data instance
   to affect kind inferece for the the data constructor signatures;
   as described at length in Note [GADT return types] in GHC.Tc.TyCl

   This led to a modest refactoring of the arguments (and argument
   order) of tcConDecl/tcConDecls.

5. Fix #19000 by inverting the sense of the test in new_locs
   in GHC.Tc.Solver.Canonical.canDecomposableTyConAppOK.

3 files changed, 37 insertions, 0 deletions

diff --git a/testsuite/tests/indexed-types/should_compile/T14111.hs b/testsuite/tests/indexed-types/should_compile/T14111.hs
new file mode 100644
index 0000000000..d1af549187
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_compile/T14111.hs
@@ -0,0 +1,24 @@
+{-# LANGUAGE MagicHash, UnboxedSums, NoImplicitPrelude #-}
+{-# LANGUAGE TypeFamilies #-}
+-- {-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE FunctionalDependencies #-}
+{-# LANGUAGE TypeInType #-}
+{-# LANGUAGE GADTs ,ExplicitNamespaces#-}
+{-# LANGUAGE UnboxedTuples #-}
+
+module T14111 where
+
+import GHC.Exts
+import GHC.Types
+import Prelude (undefined)
+import Data.Kind
+import Data.Void
+
+data family Maybe(x :: TYPE (r :: RuntimeRep))
+
+data instance Maybe (a :: Type ) where
+  MaybeSum :: (# a  | (# #) #) -> Maybe a
+
+data instance Maybe (x :: TYPE 'UnliftedRep) where
+  MaybeSumU :: (# x | (# #) #) -> Maybe x
diff --git a/testsuite/tests/indexed-types/should_compile/T8707.hs b/testsuite/tests/indexed-types/should_compile/T8707.hs
new file mode 100644
index 0000000000..379fe068b2
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_compile/T8707.hs
@@ -0,0 +1,11 @@
+{-# LANGUAGE TypeFamilies, PolyKinds, DataKinds, GADTs #-}
+
+module T8707 where
+
+import Data.Kind
+
+data family SingDF (a :: (k, k2 -> Type))
+data Ctor :: k -> Type
+
+data instance SingDF (a :: (Bool, Bool -> Type)) where
+  SFalse :: SingDF '(False, Ctor)
diff --git a/testsuite/tests/indexed-types/should_compile/all.T b/testsuite/tests/indexed-types/should_compile/all.T
index 285619f570..469dd915df 100644
--- a/testsuite/tests/indexed-types/should_compile/all.T
+++ b/testsuite/tests/indexed-types/should_compile/all.T
@@ -300,3 +300,5 @@ test('T18809', normal, compile, ['-O'])
 test('CEqCanOccursCheck', normal, compile, [''])
 test('GivenLoop', normal, compile, [''])
 test('T18875', normal, compile, [''])
+test('T8707', normal, compile, ['-O'])
+test('T14111', normal, compile, ['-O'])