diff --git a/tools/widl/header.c b/tools/widl/header.c
index 6344fb3d118..0042c639398 100644
--- a/tools/widl/header.c
+++ b/tools/widl/header.c
@@ -572,15 +572,13 @@ const var_t *is_callas(const attr_t *a)
   return get_attrp(a, ATTR_CALLAS);
 }
 
-static int write_method_macro(const type_t *iface, const char *name)
+static void write_method_macro(const type_t *iface, const char *name)
 {
-  int idx;
   func_t *cur = iface->funcs;
 
-  if (iface->ref) idx = write_method_macro(iface->ref, name);
-  else idx = 0;
+  if (iface->ref) write_method_macro(iface->ref, name);
 
-  if (!cur) return idx;
+  if (!cur) return;
   while (NEXT_LINK(cur)) cur = NEXT_LINK(cur);
 
   fprintf(header, "/*** %s methods ***/\n", iface->name);
@@ -608,13 +606,9 @@ static int write_method_macro(const type_t *iface, const char *name)
       for (c=0; c<argc; c++)
 	fprintf(header, ",%c", c+'a');
       fprintf(header, ")\n");
-      if (cur->idx == -1) cur->idx = idx;
-      else if (cur->idx != idx) yyerror("BUG: method index mismatch in write_method_macro");
-      idx++;
     }
     cur = PREV_LINK(cur);
   }
-  return idx;
 }
 
 void write_args(FILE *h, var_t *arg, const char *name, int method, int do_indent)
diff --git a/tools/widl/parser.y b/tools/widl/parser.y
index 73ccf254c7b..745056f1b7a 100644
--- a/tools/widl/parser.y
+++ b/tools/widl/parser.y
@@ -101,6 +101,8 @@ static void write_clsid(type_t *cls);
 static void write_diid(type_t *iface);
 static void write_iid(type_t *iface);
 
+static int compute_method_indexes(type_t *iface);
+
 #define tsENUM   1
 #define tsSTRUCT 2
 #define tsUNION  3
@@ -711,6 +713,7 @@ dispinterfacedef: dispinterfacehdr '{'
 						}
 /* FIXME: not sure how to handle this yet
 	| dispinterfacehdr '{' interface '}'	{ $$ = $1;
+						  compute_method_indexes($$);
 						  if (!parse_only && do_header) write_interface($$);
 						  if (!parse_only && do_idfile) write_iid($$);
 						}
@@ -737,6 +740,7 @@ interfacedef: interfacehdr inherit
 	  '{' int_statements '}'		{ $$ = $1;
 						  $$->ref = $2;
 						  $$->funcs = $4;
+						  compute_method_indexes($$);
 						  if (!parse_only && do_header) write_interface($$);
 						  if (!parse_only && do_idfile) write_iid($$);
 						}
@@ -747,6 +751,7 @@ interfacedef: interfacehdr inherit
 						  $$->ref = find_type2($3, 0);
 						  if (!$$->ref) yyerror("base class '%s' not found in import", $3);
 						  $$->funcs = $6;
+						  compute_method_indexes($$);
 						  if (!parse_only && do_header) write_interface($$);
 						  if (!parse_only && do_idfile) write_iid($$);
 						}
@@ -1539,3 +1544,26 @@ static void write_iid(type_t *iface)
   const UUID *uuid = get_attrp(iface->attrs, ATTR_UUID);
   write_guid(idfile, "IID", iface->name, uuid);
 }
+
+static int compute_method_indexes(type_t *iface)
+{
+  int idx;
+  func_t *f = iface->funcs;
+
+  if (iface->ref)
+    idx = compute_method_indexes(iface->ref);
+  else
+    idx = 0;
+
+  if (! f)
+    return idx;
+
+  while (NEXT_LINK(f))
+    f = NEXT_LINK(f);
+
+  for ( ; f ; f = PREV_LINK(f))
+    if (! is_callas(f->def->attrs))
+      f->idx = idx++;
+
+  return idx;
+}