Extend

src/Types/CommonKeys.ts

@@ -1,4 +1,4 @@
 /**
  * Extracts the common keys between two types
  */
-export type CommonKeys2<A, B> = Extract<keyof A, keyof B>
+export type CommonKeys<A, B> = Extract<keyof A, keyof B>

src/Types/Extend.ts

@@ -1,69 +1,54 @@
-import type { CommonKeys2 } from "./CommonKeys"
+import type { CommonKeys } from "./CommonKeys"
 
 
-export type Extend<T extends readonly object[]> = (
+export type Extend<Super, Self> =
+    Extendable<Super, Self> extends true
+        ? Omit<Super, CommonKeys<Self, Super>> & Self
+        : never
+
+export type Extendable<Super, Self> =
+    Pick<Self, CommonKeys<Self, Super>> extends Pick<Super, CommonKeys<Self, Super>>
+        ? true
+        : false
+
+
+export type ExtendAll<T extends readonly object[]> =
     T extends readonly [
         infer Super,
         infer Self,
         ...infer Rest extends readonly object[],
     ]
-        ? Pick<Self, CommonKeys2<Self, Super>> extends Pick<Super, CommonKeys2<Self, Super>>
-            ? Extend<readonly [
-                Omit<Super, CommonKeys2<Self, Super>> & Self,
-                ...Rest,
-            ]>
+        ? Extendable<Super, Self> extends true
+            ? ExtendAll<readonly [Extend<Super, Self>, ...Rest]>
             : never
         : T extends readonly [infer Self]
             ? Self
             : {}
-)
 
-export type Extend2<Self, Super> = Omit<Super, CommonKeys2<Self, Super>> & Self
-
-
-export type Override<T extends readonly object[]> = (
+export type ExtendableAll<T extends readonly object[]> =
     T extends readonly [
         infer Super,
         infer Self,
         ...infer Rest extends readonly object[],
     ]
-        ? Override<readonly [
-            Omit<Super, CommonKeys2<Self, Super>> & Self,
-            ...Rest,
-        ]>
-        : T extends readonly [infer Self]
-            ? Self
-            : {}
-)
-
-export type Extendable<T extends readonly object[]> = (
-    T extends readonly [
-        infer Super,
-        infer Self,
-        ...infer Rest extends readonly object[],
-    ]
-        ? Pick<Self, CommonKeys2<Self, Super>> extends Pick<Super, CommonKeys2<Self, Super>>
-            ? Extendable<readonly [
-                Omit<Super, CommonKeys2<Self, Super>> & Self,
-                ...Rest,
-            ]>
+        ? Extendable<Super, Self> extends true
+            ? ExtendableAll<readonly [Extend<Super, Self>, ...Rest]>
             : false
         : true
-)
 
-export type NonExtendableKeys<T extends readonly object[]> = (
-    T extends readonly [
-        infer Super   extends object,
-        infer Self    extends object,
-        ...infer Rest extends readonly object[],
-    ]
-        ? {[K in keyof Super & keyof Self]: Self[K] extends Super[K]
-            ? never
-            : K
-        }[keyof Super & keyof Self]
-        | NonExtendableKeys<readonly [
-            Super & Self,
-            ...Rest,
-        ]>
-        : void
-)
+// export type NonExtendableKeys<T extends readonly object[]> = (
+//     T extends readonly [
+//         infer Super   extends object,
+//         infer Self    extends object,
+//         ...infer Rest extends readonly object[],
+//     ]
+//         ? {[K in keyof Super & keyof Self]: Self[K] extends Super[K]
+//             ? never
+//             : K
+//         }[keyof Super & keyof Self]
+//         | NonExtendableKeys<readonly [
+//             Super & Self,
+//             ...Rest,
+//         ]>
+//         : void
+// )

src/Types/Merge.ts

@@ -0,0 +1,15 @@
+import type { CommonKeys } from "./CommonKeys"
+
+
+export type Merge<Super, Self> = Omit<Super, CommonKeys<Self, Super>> & Self
+
+export type MergeAll<T extends readonly object[]> =
+    T extends readonly [
+        infer Super,
+        infer Self,
+        ...infer Rest extends readonly object[],
+    ]
+        ? MergeAll<readonly [Merge<Super, Self>, ...Rest]>
+        : T extends readonly [infer Self]
+            ? Self
+            : {}

src/Types/index.ts

@@ -1,2 +1,4 @@
 export * from "./CommonKeys"
+export * from "./Extend"
+export * from "./Merge"
 export * from "./StaticType"

src/tests.ts

@@ -1,7 +1,21 @@
 import { Schema as S } from "@effect/schema"
 import { reaction, runInAction } from "mobx"
+import type { Simplify } from "type-fest"
 import { Jsonifiable, MutableTaggedClass } from "./Schema"
 import { ObservableClass } from "./Schema/MobX"
+import type { ExtendAll } from "./Types"
+
+
+type TestA = {
+    heugneu: string
+    type: "Type1" | "Type2"
+}
+
+type TestB = {
+    type: "Type1"
+}
+
+type Merged = Simplify<ExtendAll<[TestA, TestB]>>
 
 
 class User extends MutableTaggedClass<User>()("User", {