A behavior of conditional types is changed (regression)

[falsandtru]

@RyanCavanaugh @weswigham @ahejlsberg Caused by #27697

TypeScript Version: 3.4.0-dev.20190222

Search Terms:

Code

type Attrs = Record<string, string | EventListener | null | undefined>;
type Children =
  | Children.Void
  | Children.Text
  | Children.Collection
  | Children.Record;
namespace Children {
  export type Void = undefined;
  export type Text = string;
  export interface Collection extends ReadonlyArray<El> { }
  export type Record = { [field: string]: El; };
}
interface El<
  T extends string = string,
  E extends Element = Element,
  C extends Children = Children,
  > {
  readonly tag?: T;
  readonly element: E;
  children: Relax<C>;
}
type Relax<C extends Children> = C extends Children.Text ? Children.Text : C;

declare function f<C extends Children>(              children: C): C;
declare function f                    (attrs: Attrs,            ): void;
declare function f<C extends Children>(attrs: Attrs, children: C): C;
f({ a: 0 as any as El<'a', HTMLAnchorElement> }); // ok
f({ a: 0 as any as El<'a', HTMLAnchorElement, Children> }); // ok
f({ a: 0 as any as El<'a', HTMLAnchorElement, Children.Void> }); // error since 3.4.0-dev.20190222
f({ a: 0 as any as El<'a', HTMLAnchorElement, Children.Text> }); // error since 3.4.0-dev.20190222

Expected behavior:

pass

Actual behavior:

error

Playground Link: http://www.typescriptlang.org/play/index.html#src=type%20Attrs%20%3D%20Record%3Cstring%2C%20string%20%7C%20EventListener%20%7C%20null%20%7C%20undefined%3E%3B%0D%0Atype%20Children%20%3D%0D%0A%20%20%7C%20Children.Void%0D%0A%20%20%7C%20Children.Text%0D%0A%20%20%7C%20Children.Collection%0D%0A%20%20%7C%20Children.Record%3B%0D%0Anamespace%20Children%20%7B%0D%0A%20%20export%20type%20Void%20%3D%20undefined%3B%0D%0A%20%20export%20type%20Text%20%3D%20string%3B%0D%0A%20%20export%20interface%20Collection%20extends%20ReadonlyArray%3CEl%3E%20%7B%20%7D%0D%0A%20%20export%20type%20Record%20%3D%20%7B%20%5Bfield%3A%20string%5D%3A%20El%3B%20%7D%3B%0D%0A%7D%0D%0Ainterface%20El%3C%0D%0A%20%20T%20extends%20string%20%3D%20string%2C%0D%0A%20%20E%20extends%20Element%20%3D%20Element%2C%0D%0A%20%20C%20extends%20Children%20%3D%20Children%2C%0D%0A%20%20%3E%20%7B%0D%0A%20%20readonly%20tag%3F%3A%20T%3B%0D%0A%20%20readonly%20element%3A%20E%3B%0D%0A%20%20children%3A%20Relax%3CC%3E%3B%0D%0A%7D%0D%0Atype%20Relax%3CC%20extends%20Children%3E%20%3D%20C%20extends%20Children.Text%20%3F%20Children.Text%20%3A%20C%3B%0D%0A%0D%0Adeclare%20function%20f%3CC%20extends%20Children%3E(%20%20%20%20%20%20%20%20%20%20%20%20%20%20children%3A%20C)%3A%20C%3B%0D%0Adeclare%20function%20f%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20(attrs%3A%20Attrs%2C%20%20%20%20%20%20%20%20%20%20%20%20)%3A%20void%3B%0D%0Adeclare%20function%20f%3CC%20extends%20Children%3E(attrs%3A%20Attrs%2C%20children%3A%20C)%3A%20C%3B%0D%0Af(%7B%20a%3A%200%20as%20any%20as%20El%3C'a'%2C%20HTMLAnchorElement%3E%20%7D)%3B%0D%0Af(%7B%20a%3A%200%20as%20any%20as%20El%3C'a'%2C%20HTMLAnchorElement%2C%20Children%3E%20%7D)%3B%0D%0Af(%7B%20a%3A%200%20as%20any%20as%20El%3C'a'%2C%20HTMLAnchorElement%2C%20Children.Void%3E%20%7D)%3B

Related Issues:

[falsandtru]

#30010 may fix this.

[jack-williams]

Another smaller repro:

interface Foo<T> {
    prop: T extends unknown ? true : false;
}

const foo = { prop: true } as const;
const x: Foo<1> = foo;
const y: Foo<number> = foo;
const z: Foo<number> = x; // error

A smaller repro.

type Children =
    | Children.Void
    | Children.Text
    | Children.Collection
    | Children.Record;

namespace Children {
    export type Void = undefined;
    export type Text = string;
    export interface Collection extends ReadonlyArray<El> { }
    export type Record = { [field: string]: El; };
}

interface El<
    C extends Children = Children,
    > {
        children: Relax<C>;
    }

type Relax<C extends Children> = C extends Children.Text ? Children.Text : C;

const a: El = 1 as any as El<Children.Void>; // error

I think the issue here is that variance markers are marking C in Relax as invariant. This is probably caused by #27697 and the extension to variance computation.

Even though TypeScript never ends up relating two conditional types in a way the requires invariance of the check types, the effect of variance computation makes the assignment illegal.