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Inconsistent TS2345 error when using function generics

See original GitHub issue

Bug Report

🔎 Search Terms

  • Inconsistent 2345
  • function generics
  • extends 2345
  • generics 2345

🕗 Version & Regression Information

  • This changed from version 4.7.4+ and above

⏯ Playground Link

Playground link with relevant code

💻 Code

function func<T extends { foo: 1 }>(arg: T): void {}

func({ foo: 1, bar: 1 });

🙁 Actual behavior

bar property sometimes displayed as valid and sometimes produce an error:

Argument of type '{ foo: 1; bar: number; }' is not assignable to parameter of type '{ foo: 1; }'.
  Object literal may only specify known properties, and 'bar' does not exist in type '{ foo: 1; }'.(2345)

When playing with the code live and change bar to bar222 or ba sometimes the error shown and sometimes not. Refreshing the playground link with the changes always remove the error.

🙂 Expected behavior

Always display as valid without any error.

{ foo: 1, bar: 1 } is a valid subtype of { foo: 1 } and so can legally be used to instantiate T.

Maybe related to

Issue Analytics

  • State:open
  • Created a year ago
  • Comments:10 (7 by maintainers)

github_iconTop GitHub Comments

Andaristcommented, Oct 13, 2022

I was using the editor with TS built from different commits. I was doing that within the TS repo itself so I could pretty easily jump between commits/builds

fatcerberuscommented, Sep 17, 2022

Expected behavior: Always display an error for unknown properties.

The correct behavior is for there to be no error here: excess property checks aren’t intended to apply to generic constraints. { foo: 1, bar: 1 } is a valid subtype of { foo: 1 } and so can legally be used to instantiate T.

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