Join: Faster handing of readonly empty tuple (#765)

source/join.d.ts

@@ -51,7 +51,7 @@ const path: Join<['hello' | undefined, 'world' | null], '.'> = ['hello', 'world'
 export type Join<
 	Items extends readonly JoinableItem[],
 	Delimiter extends string,
-> = Items extends []
+> = Items extends readonly []
 	? ''
 	: Items extends readonly [JoinableItem?]
 		? `${NullishCoalesce<Items[0], ''>}`

test-d/join.ts

@@ -48,6 +48,11 @@ const tuple = ['foo', 'bar', 'baz'] as const;
 const joinedTuple: Join<typeof tuple, ','> = 'foo,bar,baz';
 expectType<'foo,bar,baz'>(joinedTuple);
 
+// Typeof of const empty tuple.
+const emptyTuple = [] as const;
+const joinedEmptyTuple: Join<typeof emptyTuple, ','> = '';
+expectType<''>(joinedEmptyTuple);
+
 // Typeof of string[].
 const stringArray = ['foo', 'bar', 'baz'];
 const joinedStringArray: Join<typeof stringArray, ','> = '';