Implement OVERLAPPING and OVERLAPPABLE pragmas (see #9242)

This also removes the short-lived NO_OVERLAP pragama, and renames
OVERLAP to OVERLAPS.

An instance may be annotated with one of 4 pragams, to control its
interaction with other overlapping instances:

  * OVERLAPPABLE:
    this instance is ignored if a more specific candidate exists

  * OVERLAPPING:
    this instance is preferred over more general candidates

  * OVERLAPS:
    both OVERLAPPING and OVERLAPPABLE (i.e., the previous GHC behavior).
    When compiling with -XOverlappingInstances, all instance are OVERLAPS.

  * INCOHERENT:
    same as before (see manual for details).
    When compiling with -XIncoherentInstances, all instances are INCOHERENT.

1 files changed, 10 insertions, 8 deletions

diff --git a/compiler/hsSyn/HsDecls.lhs b/compiler/hsSyn/HsDecls.lhs
index 845c05296c..313dccccd5 100644
--- a/compiler/hsSyn/HsDecls.lhs
+++ b/compiler/hsSyn/HsDecls.lhs
@@ -1055,16 +1055,18 @@ instance (OutputableBndr name) => Outputable (ClsInstDecl name) where
                map (pprDataFamInstDecl NotTopLevel . unLoc) adts ++
                pprLHsBindsForUser binds sigs ]
       where
-        top_matter = ptext (sLit "instance") <+> ppOveralapPragma mbOverlap
+        top_matter = ptext (sLit "instance") <+> ppOverlapPragma mbOverlap
                                              <+> ppr inst_ty
 
-ppOveralapPragma :: Maybe OverlapMode -> SDoc
-ppOveralapPragma mb =
+ppOverlapPragma :: Maybe OverlapMode -> SDoc
+ppOverlapPragma mb =
   case mb of
-    Nothing          -> empty
-    Just NoOverlap   -> ptext (sLit "{-# NO_OVERLAP #-}")
-    Just OverlapOk   -> ptext (sLit "{-# OVERLAP #-}")
-    Just Incoherent  -> ptext (sLit "{-# INCOHERENT #-}")
+    Nothing           -> empty
+    Just NoOverlap    -> ptext (sLit "{-# NO_OVERLAP #-}")
+    Just Overlappable -> ptext (sLit "{-# OVERLAPPABLE #-}")
+    Just Overlapping  -> ptext (sLit "{-# OVERLAPPING #-}")
+    Just Overlaps     -> ptext (sLit "{-# OVERLAPS #-}")
+    Just Incoherent   -> ptext (sLit "{-# INCOHERENT #-}")
 
 
 
@@ -1102,7 +1104,7 @@ data DerivDecl name = DerivDecl { deriv_type :: LHsType name
 
 instance (OutputableBndr name) => Outputable (DerivDecl name) where
     ppr (DerivDecl ty o)
-        = hsep [ptext (sLit "deriving instance"), ppOveralapPragma o, ppr ty]
+        = hsep [ptext (sLit "deriving instance"), ppOverlapPragma o, ppr ty]
 \end{code}
 
 %************************************************************************