>>478
> https://timsong-cpp.github.io/cppwp/n3337/expr.shift
> The behavior is undefined if the right operand is negative, or greater than
> or equal to the length in bits of the promoted left operand.

シフト演算子では右オペランドが (昇格済みの) 左オペランドの幅以上の値だったときは未定義。