Typescript: Omitted property does not narrow discriminated union contextual type

[andrewbranch]

TypeScript Version: 4.2.1

Search Terms: discriminated union optional property

Code

type DiscriminatorTrue = {
  disc: true;
  cb: (x: string) => void;
}

type DiscriminatorFalse = {
  disc?: false;
  cb: (x: number) => void;
}

type Props = DiscriminatorTrue | DiscriminatorFalse;

declare function f(options: DiscriminatorTrue | DiscriminatorFalse): any;

f({
  disc: true,
  cb: s => parseInt(s) // Inference works 👍 
});

f({
  disc: false,
  cb: n => n.toFixed() // Inference works 👍 
});

f({
  cb: n => n.toFixed() // Implicit any error 👎 
});

f({
  cb: (n: string) => parseInt(n) // But errors correctly with incorrect type annotation 👍 
});

Expected behavior:
No implicit any error on the third call site

Actual behavior:
Implicit any error on the callback parameter at the third call site

Playground Link

Related Issues: https://github.com/microsoft/TypeScript/issues/31404#issuecomment-596841068 is this issue, but the OP there is different.

Notes:
This pattern sometimes appears in React component props, where convention is to make boolean properties optional and only pass them as true (usually with the shorthand <MyComponent boolProp />). I actually suggested using discriminated union props for React components in an old blog post, and noted this issue in a footnote, calling it a possible bug, but didn’t file it at the time because I had low confidence it wasn’t a duplicate or design limitation. It was later mentioned in https://github.com/microsoft/TypeScript/issues/31404#issuecomment-596841068, but was probably ignored because it was assumed to be an instance of the OP’s issue, which was determined to be a design limitation. I dove into this again because someone tweeted at me asking about that footnote after reading the post.

[weswigham]

In this case, I think this'd be fixable by adjusting discriminateContextualTypeByObjectMembers and discriminateContextualTypeByJSXAttributes to include undefined'able discriminant props which don't appear in the node's properties. (They both currently filter the props which _do_ exist on the input object down to the discriminant ones right now, so the possibly undefined discriminants from the contextual type would need to be appended to those)

[RyanCavanaugh]

Tagging backlog since this is a longstanding shortcoming, but we can take it earlier if someone's eager to take a stab.