lists: add a subtype for valid lists

2 files changed, 4 insertions, 2 deletions

diff --git a/src/lists.adb b/src/lists.adb
index 1acbbbf38..a4585f79a 100644
--- a/src/lists.adb
+++ b/src/lists.adb
@@ -169,7 +169,7 @@ package body Lists is
       Chunk_Free_List := No_Chunk_Index;
    end Initialize;
 
-   function Iterate (List : List_Type) return Iterator
+   function Iterate (List : List_Valid_Type) return Iterator
    is
       L : List_Record renames Listt.Table (List);
    begin
diff --git a/src/lists.ads b/src/lists.ads
index a4359b51b..d06170223 100644
--- a/src/lists.ads
+++ b/src/lists.ads
@@ -24,6 +24,8 @@ package Lists is
    Null_List : constant List_Type := 0;
    List_All : constant List_Type := 1;
 
+   subtype List_Valid_Type is List_Type range List_All + 1 .. List_Type'Last;
+
    -----------
    -- Lists --
    -----------
@@ -99,7 +101,7 @@ package Lists is
    --  end loop;
    type Iterator is private;
 
-   function Iterate (List : List_Type) return Iterator;
+   function Iterate (List : List_Valid_Type) return Iterator;
    function Is_Valid (It : Iterator) return Boolean;
    procedure Next (It : in out Iterator);
    function Get_Element (It : Iterator) return El_Type;