When available, use metafield '__name' in error messages

ltm.c

@@ -1,5 +1,5 @@
 /*
-** $Id: ltm.c,v 2.35 2015/11/02 18:48:07 roberto Exp roberto $
+** $Id: ltm.c,v 2.36 2015/11/03 15:47:30 roberto Exp roberto $
 ** Tag methods
 ** See Copyright Notice in lua.h
 */
@@ -83,6 +83,22 @@ const TValue *luaT_gettmbyobj (lua_State *L, const TValue *o, TMS event) {
 }
 
 
+/*
+** Return the name of the type of an object. For tables and userdata
+** with metatable, use their '__name' metafield, if present.
+*/
+const char *luaT_objtypename (lua_State *L, const TValue *o) {
+  Table *mt;
+  if ((ttistable(o) && (mt = hvalue(o)->metatable) != NULL) ||
+      (ttisfulluserdata(o) && (mt = uvalue(o)->metatable) != NULL)) {
+    const TValue *name = luaH_getshortstr(mt, luaS_new(L, "__name"));
+    if (ttisstring(name))  /* is '__name' a string? */
+      return getstr(tsvalue(name));  /* use it as type name */
+  }
+  return ttypename(ttnov(o));  /* else use standard type name */
+}
+
+
 void luaT_callTM (lua_State *L, const TValue *f, const TValue *p1,
                   const TValue *p2, TValue *p3, int hasres) {
   ptrdiff_t result = savestack(L, p3);