Hash nil embedded pointers

src/preserves.nim

@@ -223,7 +223,7 @@ proc cannonicalize*[E](pr: var Preserve[E]) =
   else:
     discard
 
-proc hash*(pr: Preserve): Hash =
+proc hash*[E](pr: Preserve[E]): Hash =
   ## Produce a `Hash` of `pr` for use with a `HashSet` or `Table`.
   var h = hash(pr.kind.int) !& hash(pr.embedded)
   case pr.kind
@@ -254,7 +254,13 @@ proc hash*(pr: Preserve): Hash =
     for (key, val) in pr.dict.items:
       h = h !& hash(key) !& hash(val)
   of pkEmbedded:
+    when E is void:
       h = h !& hash(pr.embed)
+    else:
+      if pr.embed.isNil:
+        h = h !& hash(false)
+      else:
+        h = h !& hash(pr.embed)
   !$h
 
 proc `[]`*(pr: Preserve; i: int): Preserve =