Int constraint in Shape class for type inference

This allows the compiler to infer that this instance is used,
before knowing the type of 'i'.
The constraint will then still asure that i is an Int.

With this change, type inferencing will now correctly infer the shape of
use $ fromList (Z :. 10) [0 :: Int ..]

src/Data/Array/Accelerate/Sugar/Shape.hs

@@ -313,7 +313,12 @@ instance Shape Z where
   sliceAnyIndex  = R.SliceNil
   sliceNoneIndex = R.SliceNil
 
-instance Shape sh => Shape (sh:.Int) where
+-- Note that the constraint 'i ~ Int' allows the compiler to infer that
+-- the right argument of ':.' should be an Int.
+-- The compiler can now infer that this instance is used, before knowing
+-- the type of 'i'.
+--
+instance (Shape sh, i ~ Int) => Shape (sh:.i) where
   shapeR         = R.ShapeRsnoc (shapeR @sh)
   sliceAnyIndex  = R.SliceAll   (sliceAnyIndex  @sh)
   sliceNoneIndex = R.SliceFixed (sliceNoneIndex @sh)