Objects with the same type from different files have different behavior.

[TotooriaHyperion]

Bug Report

Objects with the same type from different files have different behavior.

It happens when the type extends a type that is not Empty.

🔎 Search Terms

Actually, the problem is so weird that as a non-native English speaker, I don't know what keyword I should use to search.

I can only find something like duplicated type declarations from multiple files etc...

🕗 Version & Regression Information

typescript: 4.4.4
tslib: 2.3.1

I just find that inconsistent behavior.

Add: Just checked all my recently used versions, this happens from(but not starts at, because I haven't used this code at versions that are lower than 4.1.3) [email protected] to [email protected].

⏯ Playground Link

It only happens in a situation with multiple files. So I have to reproduce it with a repo.

https://github.com/TotooriaHyperion/maybe-ts-bug

💻 Code

https://github.com/TotooriaHyperion/maybe-ts-bug

// We can quickly address your report if:
//  - The code sample is short. Nearly all TypeScript bugs can be demonstrated in 20-30 lines of code!
//  - It doesn't use external libraries. These are often issues with the type definitions rather than TypeScript bugs.
//  - The incorrectness of the behavior is readily apparent from reading the sample.
// Reports are slower to investigate if:
//  - We have to pare too much extraneous code.
//  - We have to clone a large repo and validate that the problem isn't elsewhere.
//  - The sample is confusing or doesn't clearly demonstrate what's wrong.

🙁 Actual behavior

Notice that in src/spec.ts, Line 25 to 35
The infer2(outerIdWithSymbol) should not error but behaves the same as infer2(innerIdWithSymbol).

// changing 
export interface ToInferWithSymbol<T = any> extends Symbol {}
// to
export interface Empty {}
export interface ToInferWithSymbol<T = any> extends Empty {}
// can fix it

// but this also has the problem
export interface NotEmpty {
  a?: number;
}
export interface ToInferWithSymbol<T = any> extends NotEmpty {}

🙂 Expected behavior

The two object(innerIdWithSymbol and outerIdWithSymbol) from different folder with same type Identifier<Target> behaves consistently.

[whzx5byb]

If you change

const innerIdWithSymbol: IdentifierWithSymbol<Target> = Symbol("");

to

const innerIdWithSymbol: IdentifierWithSymbol<Target> = Symbol("") as IdentifierWithSymbol<Target>;

It will behave the same as outerIdWithSymbol. The reason is that the type of innerIdWithSymbol is narrowed to a unique symbol, instead of IdentifierWithSymbol<Target>. There are some discussions in #46731.

PS: You can use Bug Workbench to reproduce bug with mutiple files.

[TotooriaHyperion]

It's intended behavior but needs a better hint.

related discussion
#46731

related suggestion
#45870