implicit return types for iterators are now deprecated

compiler/semtypes.nim

@@ -970,6 +970,7 @@ proc semProcTypeNode(c: PContext, n, genericParams: PNode,
   elif kind == skIterator:
     # XXX This is special magic we should likely get rid of
     r = newTypeS(tyExpr, c)
+    message(n.info, warnDeprecated, "implicit return type for 'iterator'")
 
   if r != nil:
     # turn explicit 'void' return type into 'nil' because the rest of the