import {ConditionalKeys} from 'type-fest';
interface Example {
	a: string;
	b: string | number;
	c?: string;
	d: {};
}
type StringKeysOnly = ConditionalKeys<Example, string>;

Expect to be 'a' but get 'a' | 'c' actually

export type ConditionalKeys<Base, Condition> = NonNullable<
	// Wrap in `NonNullable` to strip away the `undefined` type from the produced union.
	{
		// 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]
>;

ts version: 4.0.3

why...