Refactor default methods (Trac #11105)

This patch does some signficant refactoring to the treatment
of default methods in class declarations, and more generally
to the type checking of type/class decls.

Highlights:

* When the class has a generic-default method, such as
     class C a where
       op :: a -> a -> Bool
       default op :: Ord a => a -> a -> a
  the ClassOpItem records the type of the generic-default,
  in this case the type (Ord a => a -> a -> a)

* I killed off Class.DefMeth in favour of the very-similar
  BasicTypes.DefMethSpec.  However it turned out to be better
  to use a Maybe, thus
      Maybe (DefMethSpec Type)
  with Nothing meaning "no default method".

* In TcTyClsDecls.tcTyClGroup, we used to accumulate a [TyThing],
  but I found a way to make it much simpler, accumulating only
  a [TyCon].  Much less wrapping and unwrapping.

* On the way I also fixed Trac #10896 in a better way. Instead
  of killing off all ambiguity checks whenever there are any type
  errors (the fix in commit 8e8b9ed), I instead recover in
  TcTyClsDecls.checkValidTyCl.

There was a lot of associated simplification all round

1 files changed, 5 insertions, 4 deletions

diff --git a/compiler/iface/MkIface.hs b/compiler/iface/MkIface.hs
index 98b8830e01..d955fa5fd9 100644
--- a/compiler/iface/MkIface.hs
+++ b/compiler/iface/MkIface.hs
@@ -1529,8 +1529,9 @@ classToIfaceDecl env clas
 
     toIfaceClassOp (sel_id, def_meth)
         = ASSERT(sel_tyvars == clas_tyvars)
-          IfaceClassOp (getOccName sel_id) (toDmSpec def_meth)
+          IfaceClassOp (getOccName sel_id)
                        (tidyToIfaceType env1 op_ty)
+                       (fmap toDmSpec def_meth)
         where
                 -- Be careful when splitting the type, because of things
                 -- like         class Foo a where
@@ -1540,9 +1541,9 @@ classToIfaceDecl env clas
           (sel_tyvars, rho_ty) = splitForAllTys (idType sel_id)
           op_ty                = funResultTy rho_ty
 
-    toDmSpec NoDefMeth      = NoDM
-    toDmSpec (GenDefMeth _) = GenericDM
-    toDmSpec (DefMeth _)    = VanillaDM
+    toDmSpec :: (Name, DefMethSpec Type) -> DefMethSpec IfaceType
+    toDmSpec (_, VanillaDM)       = VanillaDM
+    toDmSpec (_, GenericDM dm_ty) = GenericDM (tidyToIfaceType env1 dm_ty)
 
     toIfaceFD (tvs1, tvs2) = (map (getFS . tidyTyVar env1) tvs1,
                               map (getFS . tidyTyVar env1) tvs2)