Non-null assertion mutates constrained type parameters (regression)

[falsandtru]

TypeScript Version: master

Search Terms:

Code

function f<T extends undefined | object>(o: T): T {
  return o!;
}

from https://github.com/falsandtru/typed-dom/blob/9e20f121ba0cc98da343a63deddeaae819d4a716/src/dom/html.ts#L36-L41

An argument o?: T is T | undefined but o! isn't T.

Expected behavior:

pass

Actual behavior:

$ node built/local/tsc.js --noEmit --strictNullChecks index.ts
index.ts(2,3): error TS2322: Type 'object' is not assignable to type 'T'.

Related Issues:

#21369?

cc @ahejlsberg @weswigham

[ajafff]

According to #21317 (comment) this is intended.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.