ConditionalKeys didn't work fine as expected

[iakoug]

trafficstars

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...

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[ClickerMonkey]

Is your project configured for strict null checking?

[iakoug]

Is your project configured for strict null checking?

... The project was forked from the repo. There was no other configuration. Is it still possible to be configured somewhere?

[sindresorhus]

This project uses strict null checking. I have confirmed the findings.

[sindresorhus]

This is really weird.

interface Example {
	a: string;
	b?: string | number;
	c?: string;
	d: Record<string, unknown>;
}

declare const exampleConditionalKeys: ConditionalKeys<Example, string>;
expectType<'a'>(exampleConditionalKeys);

Correctly results in 'a', but going through a type does not:

interface Example {
	a: string;
	b: string | number;
	c?: string;
	d: {};
}
type S = ConditionalKeys<Example, string>;
declare const exampleConditionalKeys: S;
expectType<'a'>(exampleConditionalKeys);

// @ifiokjr @kainiedziela Any ideas?

[iakoug]

This project uses strict null checking. I have confirmed the findings.

There has no "strictNullChecks": true, in tsconfig.json and "extends": "@sindresorhus/tsconfig" either. How can I confirm that.

[sindresorhus]

@sindresorhus/tsconfig has strict: true which sets strictNullChecks: true.

[iakoug]

Thanks, got it.