Finish fix for #14880.

The real change that fixes the ticket is described in
Note [Naughty quantification candidates] in TcMType.

Fixing this required reworking candidateQTyVarsOfType, the function
that extracts free variables as candidates for quantification.
One consequence is that we now must be more careful when quantifying:
any skolems around must be quantified manually, and quantifyTyVars
will now only quantify over metavariables. This makes good sense,
as skolems are generally user-written and are listed in the AST.

As a bonus, we now have more control over the ordering of such
skolems.

Along the way, this commit fixes #15711 and refines the fix
to #14552 (by accepted a program that was previously rejected,
as we can now accept that program by zapping variables to Any).

This commit also does a fair amount of rejiggering kind inference
of datatypes. Notably, we now can skip the generalization step
in kcTyClGroup for types with CUSKs, because we get the
kind right the first time. This commit also thus fixes #15743 and
 #15592, which both concern datatype kind generalisation.
(#15591 is also very relevant.) For this aspect of the commit, see
Note [Required, Specified, and Inferred in types] in TcTyClsDecls.

Test cases: dependent/should_fail/T14880{,-2},
            dependent/should_fail/T15743[cd]
            dependent/should_compile/T15743{,e}
            ghci/scripts/T15743b
            polykinds/T15592
            dependent/should_fail/T15591[bc]
            ghci/scripts/T15591

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/main/HscTypes.hs b/compiler/main/HscTypes.hs
index 445c496253..bb89c58344 100644
--- a/compiler/main/HscTypes.hs
+++ b/compiler/main/HscTypes.hs
@@ -2015,8 +2015,8 @@ tyThingParent_maybe (AConLike cl) = case cl of
     RealDataCon dc  -> Just (ATyCon (dataConTyCon dc))
     PatSynCon{}     -> Nothing
 tyThingParent_maybe (ATyCon tc)   = case tyConAssoc_maybe tc of
-                                      Just cls -> Just (ATyCon (classTyCon cls))
-                                      Nothing  -> Nothing
+                                      Just tc -> Just (ATyCon tc)
+                                      Nothing -> Nothing
 tyThingParent_maybe (AnId id)     = case idDetails id of
                                       RecSelId { sel_tycon = RecSelData tc } ->
                                           Just (ATyCon tc)