test: add cases for value-of-union ad readonly-keys-of-union

test-d/internal/readonly-keys-of-union.ts

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+import {expectType} from 'tsd';
+import type {ReadonlyKeysOfUnion} from '../../source/internal';
+
+declare const test1: ReadonlyKeysOfUnion<{readonly a: 1; b: 2}>;
+expectType<'a'>(test1);
+
+declare const test2: ReadonlyKeysOfUnion<{readonly a: 1; b?: 2} | {readonly c?: 3; d: 4}>;
+expectType<'a' | 'c'>(test2);
+
+declare const test3: ReadonlyKeysOfUnion<{readonly a: 1; b?: 2} | {readonly c?: 3; d: 4} | {readonly c: 5} | {d: 6}>;
+expectType<'a' | 'c'>(test3);
+
+// Returns `never` if there's no readonly key
+declare const test4: ReadonlyKeysOfUnion<{a: 1; b?: 2} | {c?: 3; d: 4}>;
+expectType<never>(test4);
+
+// Works with index signatures
+declare const test5: ReadonlyKeysOfUnion<{readonly [x: string]: number; a: 1} | {readonly [x: symbol]: number; a: 2}>;
+expectType<string | number | symbol>(test5);
+
+// Works with arrays
+declare const test7: ReadonlyKeysOfUnion<readonly string[] | readonly [number, number]>;
+expectType<number | typeof Symbol.unscopables | '0' | '1' | 'length'>(test7);
+
+// Works with functions
+declare const test8: ReadonlyKeysOfUnion<(() => void) | {(): void; readonly a: 1}>;
+expectType<'a'>(test8);

test-d/internal/value-of-union.ts

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+import {expectType} from 'tsd';
+import type {ValueOfUnion} from '../../source/internal';
+
+// Works with objects
+declare const test1: ValueOfUnion<{a: 1; b: 2} | {a: 3; c: 4}, 'a'>;
+expectType<1 | 3>(test1);
+
+// Works with arrays
+declare const test2: ValueOfUnion<string[] | [number, boolean], number>;
+expectType<string | number | boolean>(test2);
+
+// Works with index signatures
+declare const test3: ValueOfUnion<{[x: string]: string; a: 'a'} | {[x: number]: number; a: 'a'}, string>;
+expectType<string>(test3);
+
+declare const test4: ValueOfUnion<{[x: string]: string; a: 'a'} | {[x: number]: number; a: 'a'}, number>;
+expectType<string | number>(test4);
+
+// Works with functions
+declare const test5: ValueOfUnion<(() => void) | {(): void; a: 1} | {(): void; a: 2}, 'a'>;
+expectType<1 | 2>(test5);
+
+// Ignores objects where `Key` is missing
+declare const test6: ValueOfUnion<{a: 1; b: 2} | {a: 3; c: 4} | {a: 5; d: 6} | {e: 7}, 'a'>;
+expectType<1 | 3 | 5>(test6);
+
+// Adds `undefined` when the key is optional
+declare const test7: ValueOfUnion<{readonly a?: 1; b: 2} | {a: 3; c: 4}, 'a'>;
+expectType<1 | 3 | undefined>(test7);
+
+// Works when `Key` is a union
+declare const test8: ValueOfUnion<{a: 1; b: 2} | {a: 3; c: 4} | {a: 5; b: 6} | {e: 7}, 'a' | 'b'>;
+expectType<1 | 2 | 3 | 5 | 6>(test8);
+
+// @ts-expect-error - Errors if `Key` is missing from all of the objects
+declare const test9: ValueOfUnion<{a: 1; b: 2} | {a: 3; c: 4}, 'd'>;
+
+// Returns `any` when `Key` is `any`
+declare const test10: ValueOfUnion<{a: 1; b: 2} | {a: 3; c: 4}, any>;
+expectType<any>(test10);
+
+// Returns `never` when `Key` is `never`
+declare const test11: ValueOfUnion<{a: 1; b: 2} | {a: 3; c: 4}, never>;
+expectType<never>(test11);