add: Xor type

Author: benzaria

source/xor.d.ts

@@ -0,0 +1,95 @@
+
+/**
+Returns `false` if the giving type is `never`, else return it.
+*/
+export type IfNever<T> = [T] extends [never] ? false : T;
+
+/**
+Returns a boolean for whether either of two given types are true.
+
+Use-case: Constructing complex conditional types where multiple conditions must be satisfied.
+
+@example
+```
+import type {Xor} from 'type-fest';
+
+type TT = Xor<true, false>;
+//=> true
+
+type TF = Xor<true, false>;
+//=> true
+
+type FT = Xor<false, true>;
+//=> true
+
+type FF = Xor<false, false>;
+//=> false
+```
+
+Note: When `boolean` is passed as an argument, it is distributed into separate cases, and the final result is a union of those cases.
+For example, `Xor<false, boolean>` expands to `Xor<false, true> | Xor<false, false>`, which simplifies to `true | false` (i.e., `boolean`).
+@example
+```
+import type {Xor} from 'type-fest';
+
+type A = Xor<false, boolean>;
+//=> boolean
+
+type B = Xor<boolean, false>;
+//=> boolean
+
+type C = Xor<true, boolean>;
+//=> true
+
+type D = Xor<boolean, true>;
+//=> true
+
+type E = Xor<boolean, boolean>;
+//=> boolean
+```
+
+Note: If `never` is passed as an argument, it is treated as `false` and the result is computed accordingly.
+
+@example
+```
+import type {Xor} from 'type-fest';
+
+type A = Xor<true, never>;
+//=> true
+
+type B = Xor<never, true>;
+//=> true
+
+type C = Xor<false, never>;
+//=> false
+
+type D = Xor<never, false>;
+//=> false
+
+type E = Xor<boolean, never>;
+//=> boolean
+
+type F = Xor<never, boolean>;
+//=> boolean
+
+type G = Xor<never, never>;
+//=> false
+```
+
+@see And, Or
+*/
+export type Xor<A extends boolean, B extends boolean> = _Xor<IfNever<A>, IfNever<B>>; // `never` is treated as `false`
+
+export type _Xor<A extends boolean, B extends boolean> =
+	A extends true
+		? B extends true
+			? false
+			: true
+		: B extends true
+			? true
+			: false;
+
+type T = Xor<never, never>
+//	  ^?
+
+export {};

test-d/xor.ts

@@ -0,0 +1,32 @@
+import {expectType} from 'tsd';
+import type {Xor} from '../source/xor.d.ts';
+
+declare const boolean: boolean;
+
+expectType<Xor<true, true>>(false);
+expectType<Xor<true, false>>(true);
+expectType<Xor<false, true>>(true);
+expectType<Xor<false, false>>(false);
+
+expectType<Xor<true, boolean>>(boolean);
+expectType<Xor<boolean, true>>(boolean);
+expectType<Xor<false, boolean>>(boolean);
+expectType<Xor<boolean, false>>(boolean);
+expectType<Xor<boolean, boolean>>(boolean);
+
+// Boundary cases
+expectType<Xor<true, any>>(boolean);
+expectType<Xor<any, true>>(boolean);
+expectType<Xor<false, any>>(boolean);
+expectType<Xor<any, false>>(boolean);
+expectType<Xor<boolean, any>>(boolean);
+expectType<Xor<any, boolean>>(boolean);
+expectType<Xor<any, any>>(boolean);
+
+expectType<Xor<true, never>>(true);
+expectType<Xor<never, true>>(true);
+expectType<Xor<false, never>>(false);
+expectType<Xor<never, false>>(false);
+expectType<Xor<boolean, never>>(boolean);
+expectType<Xor<never, boolean>>(boolean);
+expectType<Xor<never, never>>(false);