Add an Eq/Ord instance for AnnotatedId.

Signed-off-by: Edward Z. Yang <[email protected]>

Cabal/Distribution/Types/AnnotatedId.hs

@@ -11,13 +11,22 @@ import Distribution.Types.ComponentName
 -- | An 'AnnotatedId' is a 'ComponentId', 'UnitId', etc.
 -- which is annotated with some other useful information
 -- that is useful for printing to users, etc.
+--
+-- Invariant: if ann_id x == ann_id y, then ann_pid x == ann_pid y
+-- and ann_cname x == ann_cname y
 data AnnotatedId id = AnnotatedId {
         ann_pid   :: PackageId,
         ann_cname :: ComponentName,
         ann_id    :: id
     }
     deriving (Show)
 
+instance Eq id => Eq (AnnotatedId id) where
+    x == y = ann_id x == ann_id y
+
+instance Ord id => Ord (AnnotatedId id) where
+    compare x y = compare (ann_id x) (ann_id y)
+
 instance Package (AnnotatedId id) where
     packageId = ann_pid