Small change in 'trymt'

Operation name can be diferent from metamethod name.

lstrlib.c

@@ -269,11 +269,18 @@ static int tonum (lua_State *L, int arg) {
 }
 
 
-static void trymt (lua_State *L, const char *mtname) {
+/*
+** To be here, either the first operand was a string or the first
+** operand didn't have a corresponding metamethod. (Otherwise, that
+** other metamethod would have been called.) So, if this metamethod
+** doesn't work, the only other option would be for the second
+** operand to have a different metamethod.
+*/
+static void trymt (lua_State *L, const char *mtkey, const char *opname) {
   lua_settop(L, 2);  /* back to the original arguments */
   if (l_unlikely(lua_type(L, 2) == LUA_TSTRING ||
-                 !luaL_getmetafield(L, 2, mtname)))
-    luaL_error(L, "attempt to %s a '%s' with a '%s'", mtname + 2,
+                 !luaL_getmetafield(L, 2, mtkey)))
+    luaL_error(L, "attempt to %s a '%s' with a '%s'", opname,
                   luaL_typename(L, -2), luaL_typename(L, -1));
   lua_insert(L, -3);  /* put metamethod before arguments */
   lua_call(L, 2, 1);  /* call metamethod */
@@ -284,7 +291,7 @@ static int arith (lua_State *L, int op, const char *mtname) {
   if (tonum(L, 1) && tonum(L, 2))
     lua_arith(L, op);  /* result will be on the top */
   else
-    trymt(L, mtname);
+    trymt(L, mtname, mtname + 2);
   return 1;
 }