Fix recursive superclasses (again). Fixes Trac #4809.

This patch finally deals with the super-delicate question of
superclases in possibly-recursive dictionaries.  The key idea
is the DFun Superclass Invariant (see TcInstDcls):

     In the body of a DFun, every superclass argument to the
     returned dictionary is
       either   * one of the arguments of the DFun,
       or       * constant, bound at top level

To establish the invariant, we add new "silent" superclass
argument(s) to each dfun, so that the dfun does not do superclass
selection internally.  There's a bit of hoo-ha to make sure that
we don't print those silent arguments in error messages; a knock
on effect was a change in interface-file format.

A second change is that instead of the complex and fragile
"self dictionary binding" in TcInstDcls and TcClassDcl,
using the same mechanism for existential pattern bindings.
See Note [Subtle interaction of recursion and overlap] in TcInstDcls
and Note [Binding when looking up instances] in InstEnv.

Main notes are here:

  * Note [Silent Superclass Arguments] in TcInstDcls,
    including the DFun Superclass Invariant

Main code changes are:

  * The code for MkId.mkDictFunId and mkDictFunTy

  * DFunUnfoldings get a little more complicated;
    their arguments are a new type DFunArg (in CoreSyn)

  * No "self" argument in tcInstanceMethod
  * No special tcSimplifySuperClasss
  * No "dependents" argument to EvDFunApp

IMPORTANT
   It turns out that it's quite tricky to generate the right
   DFunUnfolding for a specialised dfun, when you use SPECIALISE
   INSTANCE.  For now I've just commented it out (in DsBinds) but
   that'll lose some optimisation, and I need to get back to
   this.

1 files changed, 1 insertions, 1 deletions

diff --git a/compiler/simplCore/Simplify.lhs b/compiler/simplCore/Simplify.lhs
index 59c8ae4413..72227034bb 100644
--- a/compiler/simplCore/Simplify.lhs
+++ b/compiler/simplCore/Simplify.lhs
@@ -702,7 +702,7 @@ simplUnfolding :: SimplEnv-> TopLevelFlag
 simplUnfolding env _ _ _ _ (DFunUnfolding ar con ops)
   = return (DFunUnfolding ar con ops')
   where
-    ops' = map (substExpr (text "simplUnfolding") env) ops
+    ops' = map (fmap (substExpr (text "simplUnfolding") env)) ops
 
 simplUnfolding env top_lvl id _ _ 
     (CoreUnfolding { uf_tmpl = expr, uf_arity = arity