Which @angular/* package(s) are the source of the bug?

forms

Is this a regression?

No

Description

In regular object if I have a generic function with T as object union I can access shared properties:

function test<T extends { a: number; b: string } | { a: number; c: boolean }>(value: T) {
  console.log(value.a);
}

But when using FieldTree I cannot access shared properties:

function test2<T extends { a: number; b: string } | { a: number; c: boolean }>(value: FieldTree<T>) {
  console.log(value.a); // errors
}

It does work with regular union (no generics):

function test3(value: FieldTree<{ a: number; b: string } | { a: number; c: boolean }>) {
  console.log(value.a);
}

Please provide a link to a minimal reproduction of the bug

https://stackblitz.com/edit/bw2bfa5z-qkpmcxjo?file=src%2Fmain.ts

Please provide the exception or error you saw

Please provide the environment you discovered this bug in (run ng version)

Angular 21

Anything else?

No response