Opting in to getter/setter usage still fails with 'Contravariant property incompatible with covariant use'

[justin-calleja]

Flow version 0.37.4.

In the following code:

import appendQuery from 'append-query';
import { join } from 'path';
import TypeCastError from './errors/TypeCastError';

export type VersionableArgs = {
   instanceId: string,
   version: number
};

export default class Versionable {

  instanceId: string;
  version: number;
    
  constructor(args: VersionableArgs) {
    this.instanceId = args.instanceId;
    this.version = args.version;
  }

  set instanceId(val: any) {
    if (typeof val !== 'string') throw new TypeCastError('instanceId is expected to be a string');
    this.instanceId = val;
  }

  toUrl(basePath: string) {
    let x: string = join(basePath, this.instanceId);
    return appendQuery(x, {
      version: this.version
    });
  }

}

running flow check errors out with:

Potentially unsafe get/set usage. Getters and setters with side effects are potentially unsafe and disabled by default. You may opt-in to using them anyway by putting `unsafe.enable_getters_and_setters=true` into the [options] section of your .flowconfig.

Opting in to getter and setter usage still fails with:

src/Versionable.js:36
 36:     let x: string = join(basePath, this.instanceId);
                                        ^^^^^^^^^^^^^^^ property `instanceId`. Contravariant property `instanceId` incompatible with covariant use in
 36:     let x: string = join(basePath, this.instanceId);
                                        ^^^^^^^^^^^^^^^ property `instanceId`

this.instanceId is a string and join takes var args of type string… why does this error out?

I am side-stepping the issue with a // $FlowIssue comment before join usage.

[jesseschalken]

You've defined a setter but not a getter, so Flow considers instanceId to be write-only (contravariant).

If you want instanceId to be read/write from the inside and write-only from the outside, rename the real property to _instanceId and use that inside the class instead.

[justin-calleja]

@jesseschalken many thanks for the explanation. I haven't had time to try it out yet, but I will soon and close the issue then.