ConditionalKeys: Fix filtering out never type (#881)

source/conditional-keys.d.ts

@@ -1,3 +1,5 @@
+import type {IfNever} from './if-never';
+
 /**
 Extract the keys from a type where the value type of the key extends the given `Condition`.
 
@@ -36,10 +38,10 @@ export type ConditionalKeys<Base, Condition> =
 	[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
+	// Retain this key
+	// If the value for the key extends never, only include it if `Condition` also extends never
+		? IfNever<Base[Key], IfNever<Condition, Key, never>, 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];

test-d/conditional-keys.ts

@@ -6,10 +6,14 @@ type Example = {
 	b?: string | number;
 	c?: string;
 	d: Record<string, unknown>;
+	e: never;
 };
 
 declare const exampleConditionalKeys: ConditionalKeys<Example, string>;
 expectType<'a'>(exampleConditionalKeys);
 
 declare const exampleConditionalKeysWithUndefined: ConditionalKeys<Example, string | undefined>;
 expectType<'a' | 'c'>(exampleConditionalKeysWithUndefined);
+
+declare const exampleConditionalKeysTargetingNever: ConditionalKeys<Example, never>;
+expectType<'e'>(exampleConditionalKeysTargetingNever);