Bug: strictNullChecks + object spread + computed key

[bogdan]

TypeScript Version: 3.8.2

Search Terms:

• strictNullChecks
• object spread
• computed key

Code

const b = { a: 'hello' };
const key: string = 'hello';
const a: Record<string, string> = { ...b, [key]: undefined };

Expected behavior:

Type check fails

Actual behavior:

No Typecheck error.

It is interesting that if we remove explicit string type for key the code is not compilable as expected.

Playground Link:
https://www.typescriptlang.org/play/?ssl=1&ssc=1&pln=4&pc=1#code/MYewdgzgLgBARjAvDA3jAhgLhgcgBYCmANkSDjAL4DcAsAFCiSwDWBAnttAE4CWYA5klyESZWg3DQM2AEoFQXACYAebn34AaGGoEA+IWgB0xuFoDarNgF1sAVzCKCAMz4FFlcUA

[arnodb]

I may have found a related or similar or "having the same cause" or "could potentially have the same fix" bug:

interface Foo {
    a: number,
}

interface Bar {
    a?: number,
}

let foo: Foo = {
    a: 42,
};

const bar: Bar = {
    a: undefined,
}

foo = {
    ...foo,
    ...bar,
};

TSC (3.8.3 and earlier) in strict mode accepts the undefined value whereas it leads to foo not being of type Foo. Nodejs copies all entries found in bar, including the ones with an undefined value.