Omit and union types

[OliverJAsh]

Currently, if a union type is given to Omit, the returned type does not correctly preserve the union type, e.g. this test fails:

declare const foo: Omit<{ shared: string; } & ({ x: boolean } | { y: string }), 'shared'>;

/* $ExpectType Pick<{
    shared: string;
} & {
    x: boolean;
}, "x"> | Pick<{
    shared: string;
} & {
    y: string;
}, "y"> */
foo;

foo instead has type

Pick<({
    shared: string;
} & {
    x: boolean;
}) | ({
    shared: string;
} & {
    y: string;
}), never>

We can use conditional types to fix this:

export type Omit<T, K extends keyof any> = T extends any ? Pick<T, Exclude<keyof T, K>> : never;

microsoft/TypeScript#12215 (comment)

We might also want to apply this fix to some of the other helpers?

[pelotom]

@OliverJAsh sounds good, want to open a PR?