Properly represent abstract classes in Class and IfaceDecl

Summary:
Previously, abstract classes looked very much like normal
classes, except that they happened to have no methods,
superclasses or ATs, and they came from boot files.  This
patch gives abstract classes a proper representation in
Class and IfaceDecl, by moving the things which are never
defined for abstract classes into ClassBody/IfaceClassBody.

Because Class is abstract, this change had ~no disruption
to any of the code in GHC; if you ask about the methods of
an abstract class, we'll just give you an empty list.

This also fixes a bug where abstract type classes were incorrectly
treated as representationally injective (they're not!)

Fixes #13347, and a TODO in the code.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: validate

Reviewers: simonpj, bgamari, austin

Subscribers: goldfire, thomie

Differential Revision: https://phabricator.haskell.org/D3236

1 files changed, 11 insertions, 7 deletions

diff --git a/compiler/backpack/RnModIface.hs b/compiler/backpack/RnModIface.hs
index 7696d5f075..2e738c1ec6 100644
--- a/compiler/backpack/RnModIface.hs
+++ b/compiler/backpack/RnModIface.hs
@@ -451,15 +451,11 @@ rnIfaceDecl d@IfaceFamily{} = do
                      }
 rnIfaceDecl d@IfaceClass{} = do
             name <- rnIfaceGlobal (ifName d)
-            ctxt <- mapM rnIfaceType (ifCtxt d)
             binders <- mapM rnIfaceTyConBinder (ifBinders d)
-            ats <- mapM rnIfaceAT (ifATs d)
-            sigs <- mapM rnIfaceClassOp (ifSigs d)
-            return d { ifName = name
-                     , ifCtxt = ctxt
+            body <- rnIfaceClassBody (ifBody d)
+            return d { ifName    = name
                      , ifBinders = binders
-                     , ifATs = ats
-                     , ifSigs = sigs
+                     , ifBody    = body
                      }
 rnIfaceDecl d@IfaceAxiom{} = do
             name <- rnIfaceNeverExported (ifName d)
@@ -491,6 +487,14 @@ rnIfaceDecl d@IfacePatSyn{} =  do
                      , ifPatTy = pat_ty
                      }
 
+rnIfaceClassBody :: Rename IfaceClassBody
+rnIfaceClassBody IfAbstractClass = return IfAbstractClass
+rnIfaceClassBody d@IfConcreteClass{} = do
+    ctxt <- mapM rnIfaceType (ifClassCtxt d)
+    ats <- mapM rnIfaceAT (ifATs d)
+    sigs <- mapM rnIfaceClassOp (ifSigs d)
+    return d { ifClassCtxt = ctxt, ifATs = ats, ifSigs = sigs }
+
 rnIfaceFamTyConFlav :: Rename IfaceFamTyConFlav
 rnIfaceFamTyConFlav (IfaceClosedSynFamilyTyCon (Just (n, axs)))
     = IfaceClosedSynFamilyTyCon . Just <$> ((,) <$> rnIfaceNeverExported n