diff -r dd611ab202a8 -r e7e647949c95 src/HOL/Tools/Function/fun.ML
--- a/src/HOL/Tools/Function/fun.ML	Wed Jun 06 10:35:05 2012 +0200
+++ b/src/HOL/Tools/Function/fun.ML	Wed Jun 06 21:36:21 2012 +0200
@@ -84,10 +84,10 @@
     spec @ mk_catchall fixes arity_of
   end
 
-fun warnings ctxt origs tss =
+fun further_checks ctxt origs tss =
   let
-    fun warn_redundant t =
-      warning ("Ignoring redundant equation: " ^ quote (Syntax.string_of_term ctxt t))
+    fun fail_redundant t =
+      error (cat_lines ["Equation is redundant (covered by preceding clauses):", Syntax.string_of_term ctxt t])
     fun warn_missing strs =
       warning (cat_lines ("Missing patterns in function definition:" :: strs))
 
@@ -100,7 +100,7 @@
          @ ["(" ^ string_of_int (length rest) ^ " more)"])
 
     val _ = (origs ~~ tss')
-      |> map (fn (t, ts) => if null ts then warn_redundant t else ())
+      |> map (fn (t, ts) => if null ts then fail_redundant t else ())
   in
     ()
   end
@@ -119,7 +119,7 @@
       val compleqs = add_catchall ctxt fixes feqs (* Completion *)
 
       val spliteqs = Function_Split.split_all_equations ctxt compleqs
-        |> tap (warnings ctxt feqs)
+        |> tap (further_checks ctxt feqs)
 
       fun restore_spec thms =
         bnds ~~ take (length bnds) (unflat spliteqs thms)