InstanceType<T> | undefined collapses to InstanceType<T> in return type of generic method with polymorphic this

[codextremist]

🔎 Search Terms

InstanceType | undefined collapses to InstanceType

🕗 Version & Regression Information

Tested with typescript 5.9.3

⏯ Playground Link

No response

💻 Code

class Foo {
  static async test<TThis extends typeof Foo>(
    this: TThis
  ): Promise<InstanceType<TThis> | undefined> {
    return undefined
  }
}

async function test() {
const x = await Foo.test()
}

🙁 Actual behavior

type of x resolves to Foo, when it should resolve to Foo | undefined. The same happens with null

🙂 Expected behavior

It should have been resolved to Foo | undefined.

Additional information about the issue

No response

[MartinJohns]

You forgot to fully fill out the issue template (playground link missing).

I can't reproduce this, x correctly resolves to Foo | undefined. You likely have strictNullChecks disabled, in which case this would be the expected behavior.