TS2536 only when type argument is a generic type parameter

[ryan-0324]

🔎 Search Terms

ts2536, ts2536 generic, generic constraint no effect

🕗 Version & Regression Information

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

⏯ Playground Link

Playground link with relevant code

💻 Code

// Type '"foo"' cannot be used to index type 'PartialOnUndefined<T>'. (2536)
type Generic<T extends { foo: string; }> = PartialOnUndefined<T>["foo"];

// No error
type NonGeneric = PartialOnUndefined<{ foo: string; }>["foo"];

type PartialOnUndefined<T>
    = { [Key in keyof T as undefined extends T[Key] ? never : Key]: T[Key]; }
    & { [Key in keyof T as undefined extends T[Key] ? Key : never]?: T[Key]; };

🙁 Actual behavior

An error is reported when indexing PartialOnUndefined<T> with type "foo" where generic type parameter T extends { foo: string; }:

Type '"foo"' cannot be used to index type 'PartialOnUndefined<T>'. (2536)

🙂 Expected behavior

There should be no error; TS should be able to determine that "foo" can always index PartialOnUndefined<T>, since T extends { foo: string; }.

Additional information about the issue

Possibly related to #36631

Workarounds:

• Add a ts-ignore comment to silence the error; the type alias can still be used without issue.
• Explicitly narrow PartialOnUndefined<T>:

type Generic<T extends { foo: string; }>
    = PartialOnUndefined<T> extends { foo: string; }
        ? PartialOnUndefined<T>["foo"]
        : never;

[RyanCavanaugh]

I don't think we'll ever have the kind of logic that would be needed to determine that keyof PartialOnUndefined<T> was exactly the same as keyof T. Inspecting the two index signatures here to figure out that they are exactly complementary is too niche of a situation.

[typescript-bot]

This issue has been marked as "Design Limitation" and has seen no recent activity. It has been automatically closed for house-keeping purposes.