为了确保我理解正确。你是说这个按照你的预期工作：

let wideNumber: number;
const narrowNumber = wideNumber = 0; 
// const narrowNumber: 0 👍

但是这个没有：

let wideSymbol: symbol;
const narrowSymbol = wideSymbol = Symbol();
// const narrowSymbol: symbol 😕

同样，这个按照你的预期工作：

const narrowNumber = 0;
// const narrowNumber: 0
const alsoNarrowNumber = narrowNumber;
// const alsoNarrowNumber: 0 👍

但是这个没有：

const narrowSymbol = Symbol();
// const narrowSymbol: typeof narrowSymbol
const alsoNarrowSymbol = narrowSymbol;
// const alsoNarrowSymbol: symbol 😕

Playground链接
对吗？

const alsoNarrowSymbol = narrowSymbol;
// const alsoNarrowSymbol: symbol 😕
这不是我的意图，但可能是合法的。

可能有些相关。unique symbol具有一些奇怪的扩宽规则
#32242
即使是恒等函数也不安全。( <T>(t:T)=>T )

Just hit a similar widening issue to this one and the one described by @AnyhowStep on #32242 (now closed). A unique symbol ( typeof foo ) becomes just symbol when returned from a generic function of the following signature:

function reduce<T>(fn: (value: T, acc: T) => T, initial: T): T {}

String literal types are not widened by this construct, but unique symbol is. 😕