Type 'keyof T' cannot be used to index type 'T extends <type>' with unions in v4.4.4 and newer

[Basssiiie]

Bug Report

🔎 Search Terms

• Type cannot be used to index type
• keyof union type index

This one was kinda hard to find search terms for, I mainly look at changes between v.4.3.5 and v.4.4.4 in the changelog and could not find anything that could have potentially caused this.

🕗 Version & Regression Information

This changed between versions v4.3.5 and v4.4.4. It works fine on TS Playground on v4.3.5 and earlier but errors on v4.4.4 and later.

⏯ Playground Link

Playground link with relevant code

💻 Code

Same as TS Playground link minus workaround comments:

interface Button 
{
  type: "button";
  text: string;
}

interface Checkbox 
{
  type: "checkbox";
  isChecked: boolean;
}

type Control = Button | Checkbox;

function update<T extends Control, K extends keyof T>(control : T | undefined, key: K, value: T[K]): void
{
  if (control !== undefined)
  {
    control[key] = value; // this breaks on v4.4.4 but works on v4.3.5
  }
}

🙁 Actual behavior

When running the code above in v4.4.4 and later versions it errors on the last line (control[key] = value;) with:

Type 'K' cannot be used to index type 'Control'.

When transpiling the code on v4.3.5 and earlier it works fine.

Both these two workarounds work on all versions:

(control)[key] = value; // add parentheses

const c = control; // use inline variable
c[key] = value;

It seems like control is resolved to type Control in the bugged snippet but resolves to T extends Control in the other snippets.

Also sidesnotes: the error goes away if | undefined is removed from the parameter type, and also if T is changed to something that is not an union.

🙂 Expected behavior

I would expect control would resolve to T extends Control in all 3 snippets of the code.

Thank you all for your time. :)

[ahejlsberg]

This is an effect of #43183 and the subsequent revision in #44621. The revision needs to be slightly less conservative. I'm surprised no one has run into this until now.