feat(typescript-types): Make Assign assignable to Generics by removing optionals

packages/typescript-types/src/assign.ts

@@ -1,15 +1,19 @@
 import type { OptionalKeysOf } from './optional-keys-of.js'
+import type { Simplify } from './simplify.js'
+import type { PickAssignable } from './pick-assignable.js'
 
-type GetNonOptionalValueAt<Obj, Key, Default> = Key extends keyof Obj
-	? undefined extends Obj[Key]
-		? Default
-		: Obj[Key]
-	: Default
+type RemoveOptionalValues<Obj extends object> = Omit<Obj, OptionalKeysOf<Obj>>
+type RemoveRequiredKeysOfFrom<Obj1 extends object, Obj2> = Omit<Obj2, keyof RemoveOptionalValues<Obj1>>
+type MergeInto<Target, Source extends object> = RemoveRequiredKeysOfFrom<Source, Target> &
+	RemoveOptionalValues<Source>
+type MergeDefaults<
+	Shape extends object,
+	Defaults extends Pick<Required<Shape>, OptionalKeysOf<Shape>>,
+	Obj extends Shape
+> = PickAssignable<MergeInto<Defaults, Obj>, keyof Shape>
 
 export type Assign<
 	Shape extends object,
 	Defaults extends Pick<Required<Shape>, OptionalKeysOf<Shape>>,
 	Obj extends Shape
-> = {
-	[K in keyof Shape]-?: GetNonOptionalValueAt<Obj, K, K extends keyof Defaults ? Defaults[K] : never>
-}
+> = Simplify<Required<Shape> & MergeDefaults<Shape, Defaults, Obj>>

packages/typescript-types/src/is-undefined.ts

@@ -0,0 +1,9 @@
+import type { If } from './if.js'
+import type { IsAny } from './is-any.js'
+import type { IsNever } from './is-never.js'
+
+export type IsUndefined<T> = If<
+	IsAny<T>,
+	false,
+	If<IsNever<T>, false, T extends undefined ? (undefined extends T ? true : false) : false>
+>

packages/typescript-types/src/pick-assignable.ts

@@ -0,0 +1,15 @@
+import type { If } from './if.js'
+import type { IsNever } from './is-never.js'
+import type { IsUndefined } from './is-undefined.js'
+
+export type PickAssignable<T, Keys> = If<
+	IsNever<T>,
+	{},
+	If<
+		IsUndefined<T>,
+		{},
+		{
+			[K in keyof T as K extends Keys ? K : never]: T[K]
+		}
+	>
+>