Tighter parameter decorator checking with `--experimentalDecorators`

[DanielRosenwasser]

// @experimentalDecorators: true
export declare const Inject:
  (entity: Function, dataSource?: string) =>
    (target: object, key: string | symbol, index?: number) => void;

export class Foo {}

export class C {
  constructor(@Inject(Foo) x: any) {  }
}

Previously: no error
In main/5.0:

Unable to resolve signature of parameter decorator when called as an expression.
  Argument of type 'undefined' is not assignable to parameter of type 'string | symbol'

[DanielRosenwasser]

Was this...possibly always a bug with --experimentalDecorators?

[DanielRosenwasser]

Yes, it seems this was a bug in type-checking parameter decorators on constructors under --experimentalDecorators with TypeScript 4.9 and prior.

In #50820, we fixed up the way we type-checked decorators. This catches some issues, like decorators which expect a defined key; however, parameter decorators in constructors don't get passed a key. As a result, libraries with dependency injection decorator functions like inversify (pre-6.0), NestJS, and others (often named inject, Inject, and InjectRepository) might need to be corrected to accept an undefined key (or to have an overload which accepts an undefined key).

[rbuckton]

inversify's @inject works fine starting in inversify@6.0.1. @nestjs/common's @inject needs to be updated, however.

[kevinlul]

Is tsyringe affected?

[DanielRosenwasser]

Yes, I believe so.