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.

2 files changed, 17 insertions, 0 deletions

diff --git a/testsuite/tests/gadt/SynDataRec.hs b/testsuite/tests/gadt/SynDataRec.hs
new file mode 100644
index 0000000000..021ed0ba17
--- /dev/null
+++ b/testsuite/tests/gadt/SynDataRec.hs
@@ -0,0 +1,16 @@
+{-# LANGUAGE KindSignatures, DataKinds, GADTs #-}
+
+module SynDataRec where
+
+-- This mutual recursion betwen a data type and
+-- a type synonym is a little delicate.  See
+-- Note [GADT return types] in GHC.Tc.TyCl
+
+data Pass = Parsed | Renamed
+
+data GhcPass (c :: Pass) where
+  GhcPs :: GhcPs
+  GhcRn :: GhcRn
+
+type GhcPs   = GhcPass 'Parsed
+type GhcRn   = GhcPass 'Renamed
diff --git a/testsuite/tests/gadt/all.T b/testsuite/tests/gadt/all.T
index 3c146820ae..05ec39f18e 100644
--- a/testsuite/tests/gadt/all.T
+++ b/testsuite/tests/gadt/all.T
@@ -121,3 +121,4 @@ test('T15558', normal, compile, [''])
 test('T16427', normal, compile_fail, [''])
 test('T17423', expect_broken(17423), compile_and_run, [''])
 test('T18191', normal, compile_fail, [''])
+test('SynDataRec', normal, compile, [''])