`Omit` from #31115 makes all properties required

[InExtremaRes]

Since the changes of #31115, Omit<T, K> makes all properties of T required.

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TypeScript Version: 3.5.0-dev.20190501, 3.4.5

Search Terms: omit optional

Code

type A = {
    a: number;
    b?: string;
};

type B = Omit<A, 'a'>;

Expected behavior: B is { b?: string } ({ b?: string | undefined } with strictNullChecks).

Actual behavior: B is { b: string } ({ b: string | undefined } with strictNullChecks), with property b required.

Playground Link: link

pin @DanielRosenwasser

[InExtremaRes]

Just I'd like to remark that, as the playground link shows, when Omit is defined in terms of Pick the produced type preserves optional properties as optional; when it was changed to is own mapped type, all properties become required.

[jcalz]

Can we reopen #31104 then? I had some other ideas about it, if it matters