`Merge` broken after upgrade to 5.4.2

[strblr]

Bug description

The change comes from this PR: #1327

Before, you could do:

type C = Merge<A, B>;
declare let a: A;
declare let b: B;
let c: C = { ...a, ...b }

After the update, this fails with Type 'A & B' is not assignable to type 'Merge<...>.
Couldn't reproduce it in the playground.

Repro

https://www.typescriptlang.org/play/?#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

[som-sm]

This is not necessarily a bug.

When you merge two generics, TS infers the resulting type to be an intersection of both:

function merge<A, B>(a: A, b: B) {
	const c = { ...a, ...b }
	//    ^? const c: A & B
	return c;
}

Previously, A & B just happened to be assignable to Merge<A, B>, so this worked:

// With previous implementation of `Merge`
function merge<A, B>(a: A, b: B) {
	const c: Merge<A, B> = { ...a, ...b } // Fine
	return c;
}

With the newer implementation of Merge, A & B is no longer assignable to Merge<A, B>. This isn't really a problem because Merge<A, B> is not necessarily a supertype of A & B. In fact, there should exist some value of A and B such that Merge<A, B> is not a supertype of A & B:

type A = ...;
type B = ...;

declare let mergeAB: Merge<A, B>;
declare let intersectionAB: A & B;

mergeAB = intersectionAB; // This should fail for some value of `A` and `B`

Although, I have not yet been able to find such a pair.

---

Also, you shouldn't be using Merge for typing the object spread operation. Instead, use ObjectMerge for the various reasons listed over here.

I will investigate this further, but this is most likely not a bug. Meanwhile, you can use type assertion to solve this:

import type { ObjectMerge } from 'type-fest';

function merge<A extends object, B extends object>(a: A, b: B) {
	const c = { ...a, ...b } as ObjectMerge<A, B>;
	return c;
}

const m1 = merge({ a: 1, b: 1 }, { b: 'b', c: 'c' });
//    ^? const m1: { b: string; c: string; a: number; }

[strblr]

@som-sm Thanks for the explanation. I've tried ObjectMerge and settled on Merge because the former caused other problems: e.g., unassignable to constraint {} even when both sides are. Here is my use case: https://github.com/strblr/waymark/blob/e6a3c47c8feb5364aabd21be350d27f7adc5d3e7/packages/waymark/src/route.ts#L70

Btw, it would be nice to have a PartialOnUndefined utility. I've only found PartialOnUndefinedDeep in type-fest but I just need one level.

[som-sm]

@strblr

I've tried ObjectMerge and settled on Merge because the former caused other problems: e.g., unassignable to constraint {} even when both sides are.

I'd still suggest using ObjectMerge because Merge is meant for a different use case. If this is the problem you're facing, you can simply take intersection with {}:

import type { ObjectMerge } from 'type-fest';

type Test<_A extends {}> = never;

type TestAssignability<A extends {}, B extends {}> = 
    Test<ObjectMerge<A, B> & {}>; // Take intersection with `{}`

Btw, it would be nice to have a PartialOnUndefined utility. I've only found PartialOnUndefinedDeep in type-fest but I just need one level.

Please open a separate issue for the same.

[binarykitchen]

Interesting. Yesterday I raised a deepmerge bug elsewhere, here
fastify/deepmerge#82

Today I bumped type-fest from

-    "type-fest": "5.4.2",
+    "type-fest": "5.4.3",

And now this error is gone but returns a wrong type. Something odd is going around with PartialDeep. Hope this helps with your investigations?

[som-sm]

And now this error is gone but returns a wrong type.
@binarykitchen Which error, I don't see any wrong types being returned, can you please elaborate?

function merge<A extends object, B extends object>(a: A, b: B) {
	const c = {...a, ...b} as Merge<A, B>; // Same error
	return c;
}

[binarykitchen]

@som-sm Sorry, false alarm. I realised I were force-typing, which is a mistake.

I think it's more an issue about fastify/deepmerge which I've raised separately
fastify/deepmerge#82

Amazed, it's 2026, still deepmerging two objects is painful. If you don't mind, have a look at this code of mine:
https://github.com/binarykitchen/videomail-recorder/blob/main/src/features/options/reducers.ts#L19

Between Redux state changes, I am trying to deepmerge partial options into full options, yet it returns still partial options, while I expect them to remain full.

I'm amazed, it's 2026, still deepmerging is a challenge 😅
How would you deepmerge one partial object into one full object today @som-sm?