Extends: explain myself

mmkal · mmkal · commit 1e371161eeeb · 2024-08-19T12:17:04.000-04:00
diff --git a/src/utils.ts b/src/utils.ts
@@ -118,7 +118,9 @@ type ReadonlyEquivalent<X, Y> = Extends<
 >;
 
 /**
- * Checks if one type extends another.
+ * Checks if one type extends another. Note: this is not quite the same as `Left extends Right` because:
+ * 1. If either type is `never`, the result is `true` iff the other type is also `never`.
+ * 2. Types are wrapped in a 1-tuple so that union types are not distributed - instead we consider `string | number` to _not_ extend `number`. If we used `Left extends Right` directly you would get `Extends<string | number, number>` => `false | true` => `boolean`.
  */
 export type Extends<Left, Right> = IsNever<Left> extends true ? IsNever<Right> : [Left] extends [Right] ? true : false
 
