Type 'any' cannot be used as an index type silently fails when used in generics (TS2538)

🔎 Search Terms

TS2538 ts2538 silently fails

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about ts2538

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.5.2#code/C4TwDgpgBAsiAq5oF4oG0DeAzA9jgXFAM7ABOAlgHYDmAvgLpQA+UGARgIamGUCuAtmwilaAbgBQ4gPRSo8ABbkiUYaRyllAd3LB5UUJCgByDpRBGoAY1OUcwKEKi8iEACZQOy01CquIAD30kcQNoAGV5HF4AG1cAUVI1UihUOERINFMQeglpWQUlKFccCGVbe1V1D0p3InJoiEpgaJArUggOYFLq6pAQpCgAERwwnH4IXSpqAB54FX8umuU0pAA+FLlMsxz+wwB1euiAIQgAQTMN4dHxyZpplchV0SA

💻 Code

type MyType = [{foo: string}] | {bar: number};

// This errors with type 'any' cannot be used as an index type
type ShouldError = MyType[any];

// This does not error and silently creates an any
type DoSomething<T extends MyType> = T[any];
type WillBeAny = DoSomething<MyType>;

🙁 Actual behavior

When using a generic type, typescript fails to error with error TS2538 which should say that type 'any' cannot be used as an index type, instead, it does not errors and silently turns the type indexed in [any] to a any itself.

🙂 Expected behavior

I would expect typescript compilation to fail as it does when you directly access a type with any (in the example above as MyType[any] )

Additional information about the issue

This seems to be an issue from all versions I tested (several > 3.3). But I did not found an issue acknowledging this is either expected or indeed a bug.

Inizia con il TypeScript Playground collegato e la riproduzione fornita con MyType, ShouldError e DoSomething. Confronta l'accesso indicizzato generico con la gestione diretta di MyType[any] nel type checker. Il lavoro è completato quando il caso generico segnala TS2538 invece di produrre silenziosamente any, con una copertura di regressione per entrambi i casi.