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Check order dependence with mutually-recursive non-unary generics

See original GitHub issue

Bug Report

Adding an implementation of an interface allows an invalid assignability check of the interface to pass

🔎 Search Terms

assignability, unnecessary implementation

🕗 Version & Regression Information

  • This changed between versions 3.7.5 and 3.8.3

It’s still present in all versions including nightly.

⏯ Playground Link

Playground link with relevant code

Thanks @MartinJohns for the link

💻 Code

interface Parent<A, B> {
  getChild(): Child<A, B>;

  iter(): Iterable<Parent<A, B>>;

interface Child<A, B>
  extends Parent<A, B> {
  readonly a: A;
  readonly b: B;

class Impl<A, B> implements Parent<A, B> {
  constructor(readonly child: Child<A, B>) {

  getChild(): Child<A, B> {
    return this.child;

  *iter(): Iterable<Parent<A, B>> {
    const map = new Map<Child<unknown, unknown>, Child<A, B>[]>();

    function* gen(
      inp: Child<A, B>
    ): Iterable<Child<A, B>> {
      yield* map.get(inp) || [];

const x: Parent<unknown, unknown> = {} as any;
const _: Parent<null, unknown> = x; // should not pass

The final assignment should not pass. It accurately errors in 3.7.5, if the Impl class is removed (or any aspect of the iter implementation is modified), or by adding a sentinel type like sentinel?: A to the Parent interface to aid in type checking.

🙁 Actual behavior

No error is thrown in the final assignment.

🙂 Expected behavior

A n error is throw:

Type 'Parent<unknown, unknown>' is not assignable to type 'Parent<null, unknown>'.
  Type 'unknown' is not assignable to type 'null'.

Issue Analytics

  • State:open
  • Created 2 years ago
  • Reactions:3
  • Comments:9 (9 by maintainers)

github_iconTop GitHub Comments

Andaristcommented, Feb 17, 2022

I’ve spent the last 1.5 days debugging something that looks awfully like this one. This is way beyond my head and the understanding of the type system so I can’t fully assess if my issue is exactly the same or just similar. Therefore I’m going to post it, for the time being, to avoid creating a new issue that might be a duplicate of this one.

TS playground

code from the playground
declare class StateNode<TContext, TEvent extends { type: string }> {
  // if I comment out this property then error is never reported
  _storedEvent: TEvent
  // if I comment out this property then error is always reported
  _action: ActionObject<TEvent>
  // if I comment out this property then error is always reported
  _state: StateNode<TContext, any>;

interface ActionObject<TEvent extends { type: string }> {
  exec: (meta: StateNode<any, TEvent>) => void

export declare function createMachine<
  TEvent extends { type: string }
>(action: ActionObject<TEvent>): StateNode<any, any>

export declare const execute: <TEvent extends { type: string }>(
  handler: (event: TEvent) => any
) => ActionObject<TEvent>;

export declare function interpret<TContext>(
  machine: StateNode<TContext, any>
): void

// uncomment to get a correct report in the `createMachine` call below
// const test: ActionObject<{ type: "PLAY"; value: number } | { type: "RESET" }> =
//   {} as ActionObject<{
//     type: "PLAY";
//     value: number;
//   }>;

const machine = createMachine({} as any);

// comment out to get a correct report in the `createMachine` call below

createMachine<{ type: "PLAY"; value: number } | { type: "RESET" }>(
  execute((ev: { type: "PLAY"; value: number }) => {

I’ve also found out that this has changed between 3.7 and 3.8. More accurately - between 3.8.0-dev.20200117 and 3.8.0-dev.20200118. If only I’ve tracked down correctly from which commits those two were built then the behavior change is related to this diff: Given this diff, the change was introduced in this PR:

I know that at this point I’m probably just reiterating what you probably already know but since I’ve already done this detective work I’m posting this here as a reference, just in case.

I also have a hunch that another issue that I’ve posted some months ago might be related to this one: (I would have to confirm this when this one here gets fixed)

RyanCavanaughcommented, Jun 14, 2021

Simplified and cleaned up a little to make the violation more apparent

interface Parent<A> {
  child: Child<A> | null;
  parent: Parent<A> | null;

interface Child<A, B = unknown> extends Parent<A> {
  readonly a: A;
  // This field isn't necessary to the repro, but the
  // type parameter is, so including it
  readonly b: B;

function fn<A>(inp: Child<A>) {
  // This assignability check defeats the later one
  const a: Child<unknown> = inp;

// Allowed initialization of pu
const pu: Parent<unknown> = { child: { a: 0, b: 0, child: null, parent: null }, parent: null };
// Should error
const notString: Parent<string> = pu;
// Unsound read on .child.a
const m: string = notString.child!.a;
Read more comments on GitHub >

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