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.

1 files changed, 4 insertions, 10 deletions

diff --git a/testsuite/tests/typecheck/should_fail/UnliftedNewtypesFamilyKindFail2.stderr b/testsuite/tests/typecheck/should_fail/UnliftedNewtypesFamilyKindFail2.stderr
index 05f3a935eb..d609c850b7 100644
--- a/testsuite/tests/typecheck/should_fail/UnliftedNewtypesFamilyKindFail2.stderr
+++ b/testsuite/tests/typecheck/should_fail/UnliftedNewtypesFamilyKindFail2.stderr
@@ -1,11 +1,5 @@
-UnliftedNewtypesFamilyKindFail2.hs:12:20:
-     Expected a type, but ‘5’ has kind ‘GHC.Num.Natural.Natural’
-     In the first argument of ‘F’, namely ‘5’
-      In the newtype instance declaration for ‘F’
-
-UnliftedNewtypesFamilyKindFail2.hs:12:31:
-     Expected a type, but ‘5’ has kind ‘GHC.Num.Natural.Natural’
-     In the first argument of ‘F’, namely ‘5’
-      In the type ‘(F 5)’
-      In the definition of data constructor ‘MkF’
 
+UnliftedNewtypesFamilyKindFail2.hs:12:20: error:
+    • Expected a type, but ‘5’ has kind ‘GHC.Num.Natural.Natural’
+    • In the first argument of ‘F’, namely ‘5’
+      In the newtype instance declaration for ‘F’