asserts that return type is null | Generic<T> is reported as null and lose the generic

[jdeniau]

Bug Report

🔎 Search Terms

• asserts generic null
• asserts array null

🕗 Version & Regression Information

• This is the behavior in every version I tried (3.7 on the playground as asserts did appear in 3.7 IIRC), and I reviewed the FAQ for entries about null asserts

⏯ Playground Link

Playground link with relevant code

💻 Code

interface Generic<T> { value: T; }
type Ticket = {}
type TicketGeneric = null | Generic<string | Ticket>

function assertRelationIsNullOrGenericOfString(v: Generic<string | Ticket> | null): asserts v is Generic<string> | null {}

function testGeneric(ticketGeneric: TicketGeneric) {
  assertRelationIsNullOrGenericOfString(ticketGeneric)

  ticketGeneric;  // expected type is `null | Generic<string>` but gave `null`
}

🙁 Actual behavior

When an asserter return null or a Generic, it is reported as null.
It does this for every generic, even Array<string>. If the return type is not a generic, it does work fine (with null | string for exemple)

🙂 Expected behavior

It should be marked as null | Generic<string>

Thank you!

[MartinJohns]

I believe this is a duplicate of #31156. Assertion functions are essentially like type guards. Same issue is reported in #37807, which was named a duplicate. Type assertions and type guards narrowing to union types have always been wonky.

[RyanCavanaugh]

This one is actually working as intended, ish. See #35953 (comment)

[RyanCavanaugh]

This example doesn't have the subtype/assignability subtlety going on, but the general filter vs narrow behavior of UDTAs is intentional

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.