[NewErrors] 4.6.0-dev.20211126 vs 4.5.2

[typescript-bot]

The following errors were reported by 4.6.0-dev.20211126, but not by 4.5.2

lensapp/lens

4 of 5 projects failed to build with the old tsc

tsconfig.json

• error TS7023: 'steps' implicitly has return type 'any' because it does not have a return type annotation and is referenced directly or indirectly in one of its return expressions.
  • src/renderer/components/wizard/wizard.tsx#L55
• error TS7006: Parameter 'stepElem' implicitly has an 'any' type.
  • src/renderer/components/wizard/wizard.tsx#L105

[RyanCavanaugh]

I'm not 100% sure why this wasn't an error before. The loop definitely exists:

export class Wizard extends React.Component<WizardProps, State> {
  public state: State = {
    step: this.getValidStep(this.props.step), // <----
  };

  get steps() {
    const { className, title, step, header, onChange, children, ...commonProps } = this.props;
    const steps = React.Children.toArray(children) as WizardStepElem[];

    return steps.filter(step => !step.props.skip).map((stepElem, i) => {
      const stepProps = stepElem.props;

      return React.cloneElement(stepElem, {
        step: i + 1,
        wizard: this,
        next: this.nextStep,
        prev: this.prevStep,
        first: this.firstStep,
        last: this.lastStep,
        isFirst: this.isFirstStep, // <------
        isLast: this.isLastStep,
        ...commonProps,
        ...stepProps,
      } as WizardStepProps<any>);
    });
  }
  get step() {
    return this.state.step; <----
  }
  isFirstStep = () => this.step === 1; <-----

  protected getValidStep(step: number) {
    return Math.min(Math.max(1, step), this.steps.length) || 1; <----
  }

need to bisect this based on an isolated repro to see if the root cause is problematic

[RyanCavanaugh]

OK, here's the tiny version

declare const props: WizardStepProps;
export class Wizard {
  get steps() {
    return {
      wizard: this,
      ...props,
    } as WizardStepProps;
  }
}

export interface WizardStepProps {
  wizard?: Wizard;
}

[RyanCavanaugh]

Caused by #45729. When we do subtype reduction of the two contributors of wizard in the spread, which are this and Wizard. A logical workaround given those facts would be to write

    return {
      wizard: this as Wizard,
      ...props,
    } as WizardStepProps;

but this also fails in main today. I have a fix in removeSubtypes that "works" by adding an optimization to bail early on 1 types, which is troubling.