Fix ConditionalKeys - prevent never from being included unintentionally

[roryabraham]

Fixes #878

The following minimal example doesn't work as expected with the minimal example, because it includes d in the result, even though the value is never

type ConditionalKeys<Base, Condition> =
{
	// Map through all the keys of the given base type.
	[Key in keyof Base]-?:
	// Pick only keys with types extending the given `Condition` type.
	Base[Key] extends Condition
	// Retain this key since the condition passes.
		? Key
	// Discard this key since the condition fails.
		: never;

	// Convert the produced object into a union type of the keys which passed the conditional test.
}[keyof Base];

type MyMap = {
	a: {
		b: string;
	};
	c: {
		html: string;
	};
	d: never;
};

type MyFilteredMap = ConditionalKeys<MyMap, {html: string}>;

[roryabraham]

The only thing that's a bit funky with this implementation is that it doesn't work if the condition you're trying to target is never. In this example, MyFilteredMap evaluates to never, not d:

type MyMap = {
	a: {
		b: string;
	};
	c: {
		html: string;
	};
	d: never;
};

type MyFilteredMap = ConditionalKeys<MyMap, never>;

[roryabraham]

Ok, updated it to work for targeting never as well. I feel like this is more how I'd expect it to work. I might also have use-cases for that actually 🤔

[roryabraham]

let me know if you want to format this differently. I chose to progressively increase indentation for each level of ternary, and to keep comments indented to the same level as the line they're describing.

[roryabraham]

Found a relevant SO: https://stackoverflow.com/questions/58977876/generic-conditional-type-resolves-to-never-when-the-generic-type-is-set-to-never