Feature suggestion: Report unused suppressions

[AriaMinaei]

When using advanced types such as conditional types, I usually write a bunch of type tests to check for which conditions the annotations are supposed to pass/fail. Example:

As you see, every first statement checks for a condition that's supposed to pass, while every second statement checks for a condition that's correctly supposed to fail.

I'd ideally annotate those second statements with // @ts-ignore, except that if the expected type error doesn't happen, I wouldn't know about it. For example if the variable derivative is of type any, then the type annotations would obviously be wrong, but no errors would be shown.

I propose a compiler flag like warnUnusedSuppressions: boolean that would produce a lint error if a line is annotated with // @ts-ignore but doesn't contain type errors.

Related issues: #19139, #21602

[mhegazy]

This is not the intended use of // @ts-ignore. we advice that this feature be used sparingly, and I do not think we want to invest in expanding its support.

If this is a .d.ts file that you are testing, consider using https://github.com/Microsoft/dtslint, with $ExpectType assertions.

[AriaMinaei]

I see. Thanks for pointing to the correct solution!