Add Type.Assert HKT that implements behavior of TypeScript as ass…

…ertions

src/type/assert.spec.ts

@@ -0,0 +1,23 @@
+import { Test, Type } from '..'
+
+type Assert_Spec = [
+  /**
+   * Can cast a type to itself.
+   */
+  Test.Expect<Type._$assert<true, true>, true>,
+
+  /**
+   * Can cast a type to a subtype.
+   */
+  Test.Expect<Type._$assert<boolean, true>, true>,
+
+  /**
+   * Can cast a type to a supertype.
+   */
+  Test.Expect<Type._$assert<true, boolean>, boolean>,
+
+  /**
+   * Returns `never` if the types are not related.
+   */
+  Test.Expect<Type._$assert<boolean, 0>, never>
+]

src/type/assert.ts

@@ -0,0 +1,43 @@
+import { Kind } from '..'
+
+/**
+ * `_$assert` is a generic type that casts a type `T` to a type `U`, but only if `U` is a narrower or wider version of `T`.
+ * If an impossible coercion to an unrelated type is attempted, it returns `never`.
+ *
+ * This behavior is modeled after TypeScript's type assertion using the `as` operator.
+ * @see {@link Type._$cast} for a more permissive version of this type that only performs downcasts or coercions to unrelated types.
+ *
+ * @template T - The type to assert.
+ * @template U - The type to assert to.
+ *
+ * @example
+ * type T0 = _$assert<true, true> // true
+ * type T1 = _$assert<boolean, true> // true
+ * type T2 = _$assert<true, boolean> // boolean
+ * type T3 = _$assert<boolean, 0> // never
+ */
+export type _$assert<T, U> = [T] extends [U] ? U : [U] extends [T] ? U : never
+
+interface Assert_T<T> extends Kind.Kind {
+  f(x: this[Kind._]): _$assert<typeof x, T>
+}
+
+/**
+ * `Assert` is a type-level function that casts a type `T` to a type `U`, but only if `U` is a more or less specific version of `T`.
+ * If an impossible coercion to an unrelated type is attempted, it returns `never`.
+ *
+ * This behavior is modeled after TypeScript's type assertion using the `as` operator.
+ * @see {@link Type._$cast} for a more permissive version of this function that only performs downcasts or coercions to unrelated types.
+ *
+ * @template T - The type to assert.
+ * @template U - The type to assert to.
+ *
+ * @example
+ * type T0 = $<$<Assert, true>, true> // true
+ * type T1 = $<$<Assert, boolean>, true> // true
+ * type T2 = $<$<Assert, true>, boolean> // boolean
+ * type T3 = $<$<Assert, boolean>, 0> // never
+ */
+export interface Assert extends Kind.Kind {
+  f(x: this[Kind._]): Assert_T<typeof x>
+}

src/type/index.ts

@@ -1,3 +1,4 @@
+export * from './assert'
 export * from './cast'
 export * from './display'
 export * from './infer'