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, 5 insertions, 5 deletions

diff --git a/testsuite/tests/typecheck/should_fail/AssocTyDef02.stderr b/testsuite/tests/typecheck/should_fail/AssocTyDef02.stderr
index 9b3ac0e364..b310a79a6f 100644
--- a/testsuite/tests/typecheck/should_fail/AssocTyDef02.stderr
+++ b/testsuite/tests/typecheck/should_fail/AssocTyDef02.stderr
@@ -1,6 +1,6 @@
 
-AssocTyDef02.hs:6:10:
-    Type indexes must match class instance head
-    Found ‘[b]’ but expected ‘a’
-    In the type synonym instance default declaration for ‘Typ’
-    In the class declaration for ‘Cls’
+AssocTyDef02.hs:6:14:
+    Unexpected type ‘[b]’
+    In the default declaration for ‘Typ’
+    A default declaration should have form
+      default Typ a = ...