Subclass property inference fails with literal and tuple types

[shicks]

TypeScript Version: 3.8.0-dev.20191115

Search Terms:
subclass property inference string literal tuple type

Code

class Base {
  readonly x: 'a' | 'b' = 'a';
  readonly y: readonly [number, number] = [1, 2];
}

class Sub extends Base {
  x = 'b';
  y = [3, 4];
}

Expected behavior:

It should pass. The type checker should use the requirement from the base class to inform the subclass property inference, similar to how normal assignment works:

let x: 'a' | 'b' = 'a';
let y: readonly [number, number] = [1, 2];
x = 'b';
y = [3, 4];

Actual behavior:

(property) Sub.x: string
Property 'x' in type 'Sub' is not assignable to the same property in base type 'Base'.
  Type 'string' is not assignable to type '"a" | "b"'.(2416)

(property) Sub.y: number[]
Property 'y' in type 'Sub' is not assignable to the same property in base type 'Base'.
  Type 'number[]' is missing the following properties from type 'readonly [number, number]': 0, 1(2416)

Playground Link:

https://www.typescriptlang.org/play/#code/MYGwhgzhAEBCkFNoG8CwAoa0BOCwBMB7AOxAE9oAPALmgHIw7oAfegIyYF57GBuDLLgIly0MrSFFSFANrEArgFs2CbABpoC5aoC60bjICMGgEw7+6AL4YMoSDADK8ttASUALgmL4Y8CEjRMKn12OgssCgMAZg0AFnMMSyA

Related Issues:

None found

[dragomirtitian]

Properties are not contextually typed by the baseclass. The way it works is the class is typed independently and then checked against the base class.

Duplicate of: #3667
Related To: #23911, #6118, #10570

If you want tot avoid specifying the type again, using type queries could ease some of the pain:

class Base {
  x: 'a' | 'b' = 'a';
  y: readonly [number, number] = [1, 2];
}

class Sub extends Base {
  x: Base['x'] = 'b';
  y: Base['y'] = [3, 4];
}

Playground Link

[shicks]

Thanks for the reference. I guess I missed the keyword "contextual".

I'm happy to close this as a duplicate of #10570.