From eee35e7d595b4117ecf7f7775bfa90fb1962250a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Julien=20Valverd=C3=A9?= <julien.valverde@mailo.com>
Date: Mon, 19 Feb 2024 21:19:25 +0100
Subject: [PATCH] NonExtendableKeys

---
 src/tests.ts       |  4 ++--
 src/util/extend.ts | 26 +++++++++++++++++++-------
 2 files changed, 21 insertions(+), 9 deletions(-)

diff --git a/src/tests.ts b/src/tests.ts
index 09fa326..51641af 100644
--- a/src/tests.ts
+++ b/src/tests.ts
@@ -2,7 +2,7 @@ import { TraitClass } from "./Trait"
 import { trait } from "./TraitBuilder"
 import { Implements, ImplementsStatic, TraitExpressionClass } from "./TraitExpression"
 import { expression } from "./TraitExpressionBuilder"
-import { Extendable } from "./util"
+import { Extendable, NonExtendableKeys } from "./util"
 
 
 const PrintsHelloOnNew = trait
@@ -84,7 +84,7 @@ class User extends exp.extends implements Implements<typeof exp> {
 console.log(new User())
 
 
-type T = Extendable<[
+type T = NonExtendableKeys<[
     { prout: string },
     { prout: "gneugneu" },
 ]>
diff --git a/src/util/extend.ts b/src/util/extend.ts
index e4a1460..6f1c498 100644
--- a/src/util/extend.ts
+++ b/src/util/extend.ts
@@ -24,7 +24,7 @@ export type Extend<T extends readonly object[]> = (
     T extends [
         infer Super,
         infer Self,
-        ...infer Rest extends object[]
+        ...infer Rest extends object[],
     ]
         ? Pick<Self, CommonKeys<Self, Super>> extends Pick<Super, CommonKeys<Self, Super>>
             ? Extend<[
@@ -39,13 +39,13 @@ export type Extend<T extends readonly object[]> = (
 
 export type Extendable<T extends readonly object[]> = (
     T extends [
-        infer Super   extends object,
-        infer Self    extends object,
+        infer Super,
+        infer Self,
         ...infer Rest extends object[],
     ]
         ? Pick<Self, CommonKeys<Self, Super>> extends Pick<Super, CommonKeys<Self, Super>>
             ? Extendable<[
-                Super & Self,
+                Omit<Super, CommonKeys<Self, Super>> & Self,
                 ...Rest,
             ]>
             : false
@@ -53,7 +53,19 @@ export type Extendable<T extends readonly object[]> = (
 )
 
 export type NonExtendableKeys<T extends readonly object[]> = (
-    Extendable<T> extends false
-        ? {}
-        : never
+    T extends [
+        infer Super   extends object,
+        infer Self    extends object,
+        ...infer Rest extends object[],
+    ]
+        ? {
+            [K in keyof Super & keyof Self]: Self[K] extends Super[K]
+                ? never
+                : K
+        }[keyof Super & keyof Self]
+        | NonExtendableKeys<[
+            Super & Self,
+            ...Rest,
+        ]>
+        : void
 )