*  @param t2     The second type.
     *  @param site   The most derived type.
     *  @returns symbol from t2 that conflicts with one in t1.
     */
    private Symbol firstIncompatibility(DiagnosticPosition pos, Type t1, Type t2, Type site) {
        Map<TypeSymbol,Type> interfaces1 = new HashMap<TypeSymbol,Type>();
        closure(t1, interfaces1);
        Map<TypeSymbol,Type> interfaces2;
        if (t1 == t2)
            interfaces2 = interfaces1;
        else
            closure(t2, interfaces1, interfaces2 = new HashMap<TypeSymbol,Type>());

        for (Type t3 : interfaces1.values()) {
            for (Type t4 : interfaces2.values()) {
                Symbol s = firstDirectIncompatibility(pos, t3, t4, site);
                if (s != null) return s;
            }
        }
        return null;
    }