5.8.1 The operands shall be of integral or unscoped enumeration type and integral promotions are performed. The type of the result is that of the promoted left operand。 negative, or greater than or equal to the length in bits of the promoted left operand.
c++標準約定當位移右側操作數等於左側運算元的位元的個數時，運算會導致未定義行為。