Tighten checking for associated type instances

This patch finishes off Trac #11450.  Following debate on that ticket,
the patch tightens up the rules for what the instances of an
associated type can look like.  Now they must match the instance
header exactly. Eg

   class C a b where
    type T a x b

With this class decl, if we have an instance decl
  instance C ty1 ty2 where ...
then the type instance must look like
     type T ty1 v ty2 = ...
with exactly
  - 'ty1' for 'a'
  -  'ty2' for 'b', and
  - a variable for 'x'

For example:

  instance C [p] Int
    type T [p] y Int = (p,y,y)

Previously we allowed multiple instance equations and now, in effect,
we don't since they would all overlap.  If you want multiple cases,
use an auxiliary type family.

This is consistent with the treatment of generic-default instances,
and the user manual always said "WARNING: this facility (multiple
instance equations may be withdrawn in the future".

I also improved error messages, and did other minor refactoring.

1 files changed, 2 insertions, 3 deletions

diff --git a/testsuite/tests/indexed-types/should_fail/SimpleFail9.hs b/testsuite/tests/indexed-types/should_fail/SimpleFail9.hs
index 927c60caf0..9c1c4a82d2 100644
--- a/testsuite/tests/indexed-types/should_fail/SimpleFail9.hs
+++ b/testsuite/tests/indexed-types/should_fail/SimpleFail9.hs
@@ -8,8 +8,7 @@ class C7 a b where
 instance C7 Char (a, Bool) where
   data S7 (a, Bool) = S7_1
 
--- Used to fail, but now passes: 
--- type indexes don't match the instance types by name
--- but do by structure
+-- Fails because the arg to S7 should be the
+-- same as that to C7
 instance C7 Char (a, Int) where
   data S7 (b, Int) = S7_2