Extendable

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" },
+]>

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
+)