Narrowing of generic type parameter index

[Fryuni]

Bug Report

🔎 Search Terms

narrowing generic parameter

🕗 Version & Regression Information

• This changed between versions 4.4.3 and 4.6.4

⏯ Playground Link

Playground link with relevant code
Changing the version to 4.4.4 works

💻 Code

Reduced example, the code on the playground is an actual function from our codebase that used to work fine.

type TimestampColumns<T> = {
    [K in keyof T]: T[K] extends Date ? K : never
}[keyof T];

function getTimestamp<Row extends Record<string, unknown>, K extends TimestampColumns<Row>>(
  row: Row,
  timestampColumn: K,
): number {
  const date: Date = row[timestampColumn]; // This used to work;
  return date.valueOf();
}

🙁 Actual behavior

The code should compile with no warnings or errors.

🙂 Expected behavior

Fails with error Type 'Row[K]' is not assignable to type 'Date'.

[RyanCavanaugh]

This used to "work" because it was actually completely broken, e.g. in 4.4 you can also write without error:

   const date: number = row[timestampColumn];

See #48992 for a possible feature to address this

[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.