@typedly/check/lib/strict-mutual.type.d.ts

A lightweight TypeScript type definitions library for checking the types.

/**
 * @description Determines if two types `A` and `B` are **strictly and mutually** equal.
 *
 * - ✓ Checks if both types mutually extend each other: it checks if any member of `A` is assignable to `B` **AND** any member of `B` is assignable to `A`.
 * - ✓ Ensures a **definite `true` or `false` result**, avoiding `boolean` ambiguity.
 *
 * Direction:
 * - ✕ One-direction: `A extends B OR B extends A`.
 * - ✓ Bi-directional: `A extends B AND B extends A`.
 *
 * Inclusion:
 * - ✕ One-way inclusion: Checks if one type includes another.
 * - ✓ Two-way inclusion: Checks if one type includes another and another includes one.
 *
 * Mutual assignability:
 * - ✕ None.
 * - ✕ Non-tuple.
 * - ✓ Tuple-based.
 * - ✕ Function-based: Return type comparison (strict equality check).
 *
 * Distributive:
 * - ✓ Non-distributive(by using tuple): Prevents distributive behavior.
 * - ✕ Non-distributive(by using function type): Prevents distributive behavior.
 * - ✕ Distributive(without tuple): Allows distribution over unions.
 *
 * Strict:
 * - ✕ Non-strict(without tuple)(one direction): Allows subtype/supertype compatibility.
 * - ✕ Non-strict(without tuple) structural(both direction): Disallows subtype/supertype compatibility.
 * - ✓ Strict(by using tuple) structural(both direction): Ensures exact structural equality.
 * - ✕ Strict(by using function return type): Ensures exact structural equality.
 * @export
 * @template A
 * @template B
 */
export type StrictMutual<A, B> = ([A] extends [B] ? true : false) extends true ? ([B] extends [A] ? true : false) : false;
export type MutualAssignable<A, B> = StrictMutual<A, B>;
export type BooleanStructuralEqual<A, B> = StrictMutual<A, B>;