添加新属性时,ts(1170)映射类型出错

我有以下代码:

enum A {
    FOO = 0,
    BAR = 1,
    BAZ = 2
}
type B = {
    [key in A]: number
}
declare let b: B;

很好用。我可以用 b[A.FOO] b[123] . 但是,我也希望能够使用 b.length ,但当我向类型添加属性时 B

type B = {
    length: number,
    [key in A]: number
}

…我得到以下错误

A computed property name in a type literal must refer to an expression whose type is a literal type or a 'unique symbol' type.ts(1170)
A computed property name must be of type 'string', 'number', 'symbol', or 'any'.ts(2464)

我怎样才能解决这个问题?