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, 8 insertions, 4 deletions

diff --git a/compiler/utils/Binary.hs b/compiler/utils/Binary.hs
index 82d1497ee6..0aa8c648b8 100644
--- a/compiler/utils/Binary.hs
+++ b/compiler/utils/Binary.hs
@@ -834,15 +834,19 @@ instance Binary RecFlag where
               _ -> do return NonRecursive
 
 instance Binary OverlapMode where
-    put_ bh NoOverlap   = putByte bh 0
-    put_ bh OverlapOk   = putByte bh 1
-    put_ bh Incoherent  = putByte bh 2
+    put_ bh NoOverlap     = putByte bh 0
+    put_ bh Overlaps      = putByte bh 1
+    put_ bh Incoherent    = putByte bh 2
+    put_ bh Overlapping   = putByte bh 3
+    put_ bh Overlappable  = putByte bh 4
     get bh = do
         h <- getByte bh
         case h of
             0 -> return NoOverlap
-            1 -> return OverlapOk
+            1 -> return Overlaps
             2 -> return Incoherent
+            3 -> return Overlapping
+            4 -> return Overlappable
             _ -> panic ("get OverlapMode" ++ show h)