Legacy/Experimental parameter decorators fails with TS1239 when StrictNullChecks is true in Typescript 5.0.2

[mrttrifork]

Bug Report

🔎 Search Terms

experimentalDecorator, TS1239, parameter decorator

🕗 Version & Regression Information

4.8.3 -> 5.0.2

⏯ Playground Link

Playground link with relevant code

💻 Code

I provided the playground link

🙁 Actual behavior

i assume @Dependency("test) returns undefined and i set the type to string so i assume that's why it fails now

🙂 Expected behavior

There should be no error

[notaphplover]

It seems legacy decorators now requires their second parameter to allow undefined values:

export function Dependency(injectable: ReferenceType<unknown>) {
    return function (target: any, _: string | undefined, parameterIndex: number) {
       ...
    }
}

Consider this example.

Having said that, it would be great to update legacy decorator typings to reflect this change. This is breaking every single decorator that relies on ParameterDecorator in ts5 with strict null checks. Consider this example.

cc @rbuckton

[mrttrifork]

Thanks a lot i added

export type UpdatedLegacyParameterDecorator = (target: Object, propertyKey: string | undefined, parameterIndex: number) => void;

as a workaround

[rbuckton]

The reported issue is an expected behavior and is mentioned in the breaking changes section of the 5.0 release blog post: https://devblogs.microsoft.com/typescript/announcing-typescript-5-0/#more-accurate-type-checking-for-parameter-decorators-in-constructors-under-experimentaldecorators

@notaphplover is correct that the typings for the legacy ParameterDecorator type do not reflect this change.