Entirely re-jig the handling of default type-family instances (fixes Trac #9063)

In looking at Trac #9063 I decided to re-design the default
instances for associated type synonyms.  Previously it was all
jolly complicated, to support generality that no one wanted, and
was arguably undesirable.

Specifically

* The default instance for an associated type can have only
  type variables on the LHS.  (Not type patterns.)

* There can be at most one default instances declaration for
  each associated type.

To achieve this I had to do a surprisingly large amount of refactoring
of HsSyn, specifically to parameterise HsDecls.TyFamEqn over the type
of the LHS patterns.

That change in HsDecls has a (trivial) knock-on effect in Haddock, so
this commit does a submodule update too.

The net result is good though.  The code is simpler; the language
specification is simpler.  Happy days.

Trac #9263 and #9264 are thereby fixed as well.

1 files changed, 6 insertions, 0 deletions

diff --git a/testsuite/tests/polykinds/T9264.hs b/testsuite/tests/polykinds/T9264.hs
new file mode 100644
index 0000000000..df75599e56
--- /dev/null
+++ b/testsuite/tests/polykinds/T9264.hs
@@ -0,0 +1,6 @@
+{-# LANGUAGE PolyKinds, TypeFamilies, ScopedTypeVariables #-}
+module T9264 where
+
+class C (a :: k) where
+   type F (a :: k)
+   type F (a :: k) = Int