Assignment across the same type fails in TS 3.5

[domoritz]

I found an issue with assigning from one object to another when the types and key are the same. This issue did not occur in TS 3.4.

I understand that this is fixing unsound assignments but I wonder what the right pattern is now.

TypeScript Version: 3.5

Search Terms: keyof, unsound, assigment

Code

type D = {
    foo?: number,
    bar?: string,
};

const a: D = {};
const b: D = {};

const k = "foo" as keyof D;

a[k] = b[k];

Expected behavior:

No error

Actual behavior:

Type 'string | number | undefined' is not assignable to type 'undefined'.
  Type 'string' is not assignable to type 'undefined'.

I have to write this to fix the issue (is there a better way?)

type D = {
    foo?: number,
    bar?: string,
};

const a: D = {};
const b: D = {};

const k = "foo" as keyof D;

a[k] = b[k] as any;

Playground Link:

Related Issues:

[RyanCavanaugh]

The safer version would be

(a[k] as D[keyof D]) = b[k];

This is probably an artifact of your repro reduction, but the cast-free initialization of k also checks without error:

const k: keyof D = "foo";
a[k] = b[k];

[domoritz]

Ahh, I see.

The specific use case for me was in an iteration over the keys of an object.

for (const k of Object.keyof(b) as (keyof D)[]) {
  a[k] = b[k];
}

Is there a different pattern that would make this code valid?

[domoritz]

I think may be related to problems with upgrading to TS 3.5 in vega/vega-lite#5005 (details).