Can't infer unique symbol types via another variable

[falsandtru]

Literals such as 0 can be inferred correctly but unique symbols are not.

TypeScript Version: 3.7.x-dev.20191224

Search Terms:

Code

let revision: symbol;
const rev = revision = Symbol();
rev === revision;

Expected behavior:
Type rev is unique symbol.
Actual behavior:
Type rev is symbol. Can't even declare that rev is a unique symbol.
Playground Link: https://www.typescriptlang.org/play/index.html?ts=3.8.0-dev.20191224&ssl=1&ssc=1&pln=3&pc=18#code/DYUwLgBATiBuCWBneB7AdgLgogngWwCMVgBuAKAGN1FIZYIBeaOJVNRiAZXyOAAoAlOTqMGTOq3QkgA

Related Issues:

[jcalz]

Just to make sure I understand. You're saying this works as you expect:

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

but this doesn't:

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

---

Similarly, this works as you expect:

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

but this doesn't:

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

Playground Link

Right?

[falsandtru]

const alsoNarrowSymbol = narrowSymbol;
// const alsoNarrowSymbol: symbol 😕

Not my intent but probably legal.

[AnyhowStep]

Probably somewhat related. unique symbol has some funky widening rules

#32242

Even identity functions aren't safe from them. (<T>(t:T)=>T)

[coreh]

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. 😕