Type inference for union of arrays broken

[ulrichb]

TypeScript Version: Version 3.1.0-dev.20180925

Search Terms: array this context type compatibility assignability

Code

interface Array<T> {
    equalsShallow<T>(this: ReadonlyArray<T>, other: ReadonlyArray<T>): boolean;
}

type ExtractArrayType<TArray> = TArray extends Array<infer T> ? T : never;

[
    { a: ["x"], b: [] },
    { a: [], b: ["x"] },
    { a: ["x", 2], b: ["x", 2] },
    { a: ["1"], b: [1] },
    { a: [1], b: ["1"] },
    { a: [null], b: [undefined] },
    { a: [undefined], b: [null] },
    { a: [{}], b: [{}] },
].forEach(({ a, b }) => {

    // Compiles:
    const assignedA: typeof a = b;
    const assignedB: typeof b = a;

    // Compiles (prove that ReadonlyArray<>-types are compatible):
    const assignedAReadonly: ReadonlyArray<ExtractArrayType<typeof a>> = b;
    const assignedBReadonly: ReadonlyArray<ExtractArrayType<typeof b>> = a;

    // error TS2684: The 'this' context of type '(string | number)[] | null[] | undefined[] | {}[]'
    // is not assignable to method's 'this' of type 'ReadonlyArray<string | number>':
    a.equalsShallow(b);
});

Works in TS 3.0.

Note that the error disappears when changing equalsShallow() to equalsShallow<T>(this: Array<T>, other: Array<T>): boolean.

Further note that the error disappears when extracting a type for a and b (instead of using the literal type inference).

Expected behavior: No compile error

Actual behavior: Compile error, see snippet

Playground Link: Link (no error because it's TS 3.0)

Simplified repro:

declare function f<T>(a: ReadonlyArray<T>, other: ReadonlyArray<T>): boolean;
declare const a: number[] | string[];
f(a, a);

[ahejlsberg]

This appears to be an effect of #27028 which I merged yesterday. The release-3.1 branch is not affected.

[ulrichb]

Okay, If I understood it correctly, the 3.2 feature #27028 was on the 3.1 (@next) channel for short amount of time and introduced this issue => this issue won't arrive in the 3.1 release and therefore is tagged with the 3.2 milestone.

Correct?

[ahejlsberg]

@Andy-MS Your simplified repro above is already an error in 3.0. This is a correct simplified repro:

declare function f<T>(a: ReadonlyArray<T>, other: ReadonlyArray<T>): boolean;
declare const a: number[] | {}[];
f(a, a);

@ulrichb The upcoming 3.1 release is in the release-3.1 branch, so master is technically the 3.2 release now (and has been for several days). We just hadn't bumped the version tag (but that's done now).