Comments to explain strict overlap checking for type family instances

diff --git a/compiler/typecheck/FamInst.lhs b/compiler/typecheck/FamInst.lhs
index 89dffbf..67c526d 100644 (file)
--- a/compiler/typecheck/FamInst.lhs
+++ b/compiler/typecheck/FamInst.lhs
@@ -36,6 +36,16 @@ check whether the instances in the two modules are consistent, *unless* we can
 be certain that the instances of the two modules have already been checked for
 consistency during the compilation of modules that we import.
 
 be certain that the instances of the two modules have already been checked for
 consistency during the compilation of modules that we import.
 

+Why do we need to check?  Consider 
+   module X1 where               module X2 where
+    data T1                        data T2
+    type instance F T1 b = Int     type instance F a T2 = Char
+    f1 :: F T1 a -> Int                    f2 :: Char -> F a T2
+    f1 x = x                       f2 x = x
+
+Now if we import both X1 and X2 we could make (f2 . f1) :: Int -> Char.
+Notice that neither instance is an orphan.
+

 How do we know which pairs of modules have already been checked?  Any pair of
 modules where both modules occur in the `HscTypes.dep_finsts' set (of the
 `HscTypes.Dependencies') of one of our directly imported modules must have
 How do we know which pairs of modules have already been checked?  Any pair of
 modules where both modules occur in the `HscTypes.dep_finsts' set (of the
 `HscTypes.Dependencies') of one of our directly imported modules must have