Instance declaration overlap allowed if *either* has -XOverlappingInstances

[ghc-hetmet.git] / compiler / types / InstEnv.lhs

diff --git a/compiler/types/InstEnv.lhs b/compiler/types/InstEnv.lhs
index dfbe0a0..34bd5eb 100644 (file)
--- a/compiler/types/InstEnv.lhs
+++ b/compiler/types/InstEnv.lhs
@@ -533,14 +533,18 @@ insert_overlapping new_item (item:items)
     old_beats_new = item `beats` new_item
 
     (instA, _) `beats` (instB, _)
-       = overlap_ok && 
-         isJust (tcMatchTys (is_tvs instB) (is_tys instB) (is_tys instA))
-               -- A beats B if A is more specific than B, and B admits overlap
-               -- I.e. if B can be instantiated to match A
-       where
-         overlap_ok = case is_flag instB of
-                       NoOverlap -> False
-                       _         -> True
+          = overlap_ok && 
+            isJust (tcMatchTys (is_tvs instB) (is_tys instB) (is_tys instA))
+                    -- A beats B if A is more specific than B,
+                    -- (ie. if B can be instantiated to match A)
+                    -- and overlap is permitted
+          where
+            -- Overlap permitted if *either* instance permits overlap
+            -- This is a change (Trac #3877, Dec 10). It used to
+            -- require that instB (the less specific one) permitted overlap.
+            overlap_ok = case (is_flag instA, is_flag instB) of
+                              (NoOverlap, NoOverlap) -> False
+                              _                      -> True
 \end{code}