diff --git a/src/lib/es2015.core.d.ts b/src/lib/es2015.core.d.ts
index 5d570bb0866..2373b828fdd 100644
--- a/src/lib/es2015.core.d.ts
+++ b/src/lib/es2015.core.d.ts
@@ -278,7 +278,7 @@ interface ObjectConstructor {
       * @param target The target object to copy to.
       * @param source The source object from which to copy properties.
       */
-    assign<T, U>(target: T, source: U): T & U;
+    assign<T, U>(target: T, source: U): { ...T, ...U };
 
     /**
       * Copy the values of all of the enumerable own properties from one or more source objects to a
@@ -287,7 +287,7 @@ interface ObjectConstructor {
       * @param source1 The first source object from which to copy properties.
       * @param source2 The second source object from which to copy properties.
       */
-    assign<T, U, V>(target: T, source1: U, source2: V): T & U & V;
+    assign<T, U, V>(target: T, source1: U, source2: V): { ...T, ...U, ...V };
 
     /**
       * Copy the values of all of the enumerable own properties from one or more source objects to a
@@ -297,7 +297,7 @@ interface ObjectConstructor {
       * @param source2 The second source object from which to copy properties.
       * @param source3 The third source object from which to copy properties.
       */
-    assign<T, U, V, W>(target: T, source1: U, source2: V, source3: W): T & U & V & W;
+    assign<T, U, V, W>(target: T, source1: U, source2: V, source3: W): { ...T, ...U, ...V, ...W };
 
     /**
       * Copy the values of all of the enumerable own properties from one or more source objects to a