Fix Object.assign

Author: graphemecluster

lib/lib.es2015.core.d.ts

@@ -74,7 +74,7 @@ interface ObjectConstructor {
   assign<T, Ts extends readonly any[]>(
     target: T,
     ...sources: Ts
-  ): UnionToIntersection<T | Ts[number]>;
+  ): First<UnionToIntersection<[T] | { [K in keyof Ts]: [Ts[K]] }[number]>>;
 
   /**
    * Returns an array of all symbol properties found directly on object o.

lib/lib.es5.d.ts

@@ -1,3 +1,5 @@
+type First<T> = T extends [any] ? T[0] : unknown;
+
 type UnionToIntersection<T> = (
   T extends any ? (arg: T) => void : never
 ) extends (arg: infer F) => void

lib/lib.util.d.ts

@@ -1,3 +1,5 @@
+type First<T> = T extends [any] ? T[0] : unknown;
+
 type UnionToIntersection<T> = (
   T extends any ? (arg: T) => void : never
 ) extends (arg: infer F) => void