Qt Creator doesn't seem to know about a few Qt Quick 2.0 basic types. Using the one of the types vector2d, vector4d, quaternion or matrix4x4 as property type causes it to be marked as error M304. The error also shows up when checks (Tools->QML/JS->Run Checks) are run. The full error text is Invalid property type 'vector2d'. (M304).

Example:

import QtQuick 2.0

Item {
	property vector2d v
}