Type inference for union of arrays broken #27337
Description
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: { // Works: const assignedA: typeof a = b; const assignedB: typeof b = a; // Prove that ReadonlyArray<>-types are compatible: const assignedAReadonly: ReadonlyArray