Typescript Type Resolution Fails with generics of T | null #62608

🔎 Search Terms

type indexing generics vscode intellisense resolution

🕗 Version & Regression Information

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

Note: While i've provided the playground link, the playground doesnt show type intellisense the same as vscode does, and so doesnt actually show a difference.

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAygxgQwDYIE4GcoF4oG8BQUUAcgK4C2ARhKgFxQB2F1qA3PgL7v6iRQCyCENQA8AFQB82KGKgAfRqSRJuvaGPAQAgtIJEE9eMjToA2gCIyVGuYC68xcvZFK9QcIgim11BOdQ4Q0QUDAsrFjsHJidOVU0ZTQAhXUIoA1hgkzDmG3sFaJVU10UfKKVCokCM41DLHNRI-PL2LiA

💻 Code

type Scalars = {
  Number: number;
};

type Maybe<T> = T | null;

type TypeA = {
  a: Scalars["Number"] | null;
  b: Maybe<number>;
  c: Scalars["Number"] | null;
};

type TypeB = {
  a: Scalars["Number"] | null;
  b: number | null;
  c: Scalars["Number"] | null;
};

🙁 Actual behavior

I'm not sure if this is a typescript issue or a vscode issue.

In VSCode with typescript 5.9.3 installed with just a single file called typeissue.ts and the above code in it.

If you have a generic type definition that is:

type Maybe<T> = T | null;

And you use it in a type, any types after that generic usage that are using type indexing, will no longer correctly resolve to their types.

Hovering over TypeA will show:

Where you can see the "c" field doesnt get "resolved" to number | null

Hovering over TypeB will show:

Where you can see all fields are shown as number | null even when using type indexing.

🙂 Expected behavior

TypeA should show exactly the same as TypeB.

Additional information about the issue

This was found in a React + Typescript project we have, that is using React-Hook-Forms and graphql codegen types.
Where the types use type indexing, and we were trying to use the hook useFieldArray.
VSCode Typescript shows an error that there are no field arrays in the type.