From b9dc68d97bb1d3f9c7e6a32d71690bd4e4e2ea12 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Julien=20Valverd=C3=A9?= <julien.valverde@mailo.com>
Date: Mon, 19 Feb 2024 18:23:09 +0100
Subject: [PATCH] Extendable

---
 src/tests.ts       |  7 +++++++
 src/util/extend.ts | 29 ++++++++++++++++++-----------
 2 files changed, 25 insertions(+), 11 deletions(-)

diff --git a/src/tests.ts b/src/tests.ts
index 631b011..09fa326 100644
--- a/src/tests.ts
+++ b/src/tests.ts
@@ -2,6 +2,7 @@ import { TraitClass } from "./Trait"
 import { trait } from "./TraitBuilder"
 import { Implements, ImplementsStatic, TraitExpressionClass } from "./TraitExpression"
 import { expression } from "./TraitExpressionBuilder"
+import { Extendable } from "./util"
 
 
 const PrintsHelloOnNew = trait
@@ -81,3 +82,9 @@ class User extends exp.extends implements Implements<typeof exp> {
 }
 
 console.log(new User())
+
+
+type T = Extendable<[
+    { prout: string },
+    { prout: "gneugneu" },
+]>
diff --git a/src/util/extend.ts b/src/util/extend.ts
index b8f2e95..d2a3824 100644
--- a/src/util/extend.ts
+++ b/src/util/extend.ts
@@ -21,18 +21,25 @@ import { CommonKeys } from "."
 
 
 export type Extend<T extends readonly object[]> = (
-    T extends [infer Super, infer Self, ...infer Rest]
-        ? Rest extends object[]
-            ? Pick<Self, CommonKeys<Self, Super>> extends Pick<Super, CommonKeys<Self, Super>>
-                ? Extend<[
-                    Omit<Super, CommonKeys<Self, Super>> & Self,
-                    ...Rest,
-                ]>
-                : never
+    T extends [infer Super, infer Self, ...infer Rest extends object[]]
+        ? Pick<Self, CommonKeys<Self, Super>> extends Pick<Super, CommonKeys<Self, Super>>
+            ? Extend<[
+                Omit<Super, CommonKeys<Self, Super>> & Self,
+                ...Rest,
+            ]>
             : never
         : T extends [infer Self]
-            ? Self extends object
-                ? Self
-                : never
+            ? Self
             : {}
 )
+
+export type Extendable<T extends readonly object[]> = (
+    T extends [infer Super, infer Self, ...infer Rest extends object[]]
+        ? Pick<Self, CommonKeys<Self, Super>> extends Pick<Super, CommonKeys<Self, Super>>
+            ? Extendable<[
+                Omit<Super, CommonKeys<Self, Super>> & Self,
+                ...Rest,
+            ]>
+            : false
+        : true
+)