ConditionalKeys does not correctly filter out never type

[roryabraham]

I'm seeing some unexpected behavior in this minimal example:

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}>;

The root cause is that never can extend anything. I'm not sure if this is expected behavior or not, but it was a gotcha for me. It can be fixed like so:

type ConditionalKeys<Base, Condition> = {
    [Key in keyof Base]-?:
        // The key is retained if its value extends the given `Condition` and is not `never`
        Base[Key] extends Condition
            ? (Base[Key] extends never ? never : Key)
            : never;
}[keyof Base];

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[roryabraham]

Happy to open a PR if we agree that this is unexpected behavior.

[sindresorhus]

That is indeed unexpected behavior. A pull request would be amazing 🙏

[roryabraham]

Opened #881. Thanks in advance for reviewing 🙇🏼