Refactor named wildcards (again)

Michal's work on #10982, #11098, refactored the handling of named
wildcards by making them more like ordinary type variables.

This patch takes the same idea to its logical conclusion, resulting
in a much tidier, tighter implementation.

Read Note [The wildcard story for types] in HsTypes.

Changes:

 * Named wildcards are ordinary type variables, throughout

 * HsType no longer has a data constructor for named wildcards
   (was NamedWildCard in HsWildCardInfo).  Named wildcards are
   simply HsTyVars

 * Similarly named wildcards disappear from Template Haskell

 * I refactored RnTypes to avoid polluting LocalRdrEnv with something
   as narrow as named wildcards.  Instead the named wildcard set is
   carried in RnTyKiEnv.

There is a submodule update for Haddock.

2 files changed, 12 insertions, 6 deletions

diff --git a/testsuite/tests/partial-sigs/should_fail/NamedExtraConstraintsWildcard.hs b/testsuite/tests/partial-sigs/should_fail/NamedExtraConstraintsWildcard.hs
index 28bb8362ce..c9a00f33cc 100644
--- a/testsuite/tests/partial-sigs/should_fail/NamedExtraConstraintsWildcard.hs
+++ b/testsuite/tests/partial-sigs/should_fail/NamedExtraConstraintsWildcard.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE PartialTypeSignatures, NamedWildCards #-}
+{-# LANGUAGE ConstraintKinds, PartialTypeSignatures, NamedWildCards #-}
 module NamedExtraConstraintsWildcard where
 
 foo :: (Eq a, _a) => a -> a
diff --git a/testsuite/tests/partial-sigs/should_fail/NamedExtraConstraintsWildcard.stderr b/testsuite/tests/partial-sigs/should_fail/NamedExtraConstraintsWildcard.stderr
index 47e9b99135..bb1a481a1e 100644
--- a/testsuite/tests/partial-sigs/should_fail/NamedExtraConstraintsWildcard.stderr
+++ b/testsuite/tests/partial-sigs/should_fail/NamedExtraConstraintsWildcard.stderr
@@ -1,5 +1,11 @@
-

-NamedExtraConstraintsWildcard.hs:4:15: error:

-    Named wildcard ‘_a’ not allowed as an extra-contraint

-      Use an anonymous wildcard instead

-      in the type signature for ‘foo’

+
+NamedExtraConstraintsWildcard.hs:5:1: error:
+    • Could not deduce: t0
+      from the context: (Eq a, t)
+        bound by the inferred type for ‘foo’:
+                   (Eq a, t) => a -> a
+        at NamedExtraConstraintsWildcard.hs:5:1-15
+    • In the ambiguity check for the inferred type for ‘foo’
+      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
+      When checking the inferred type
+        foo :: forall a (t :: Constraint). (Eq a, t) => a -> a