Bug with type argument inference if it has default value = undefined and actual value is object with non-arrow function

There is a difference in how TypeScript infers the type automatically based on type of the argument we pass in:

how the value is declared (inline or by reference)
whether the value is an inline object with non-arrow function

The error is reproduced if generic parameter has default value undefined (this is mandatory in my case.)

🔎 Search Terms

"generics in constructor"
"type argument inference undefined"

🕗 Version & Regression Information

This is the behavior in every version I tried from 3.9.7 to 5.9.3

⏯ Playground Link

💻 Code

This issue with function:

function make<T, S=T, D=undefined>(value: T, options: NewOptions<T, S, D>) {
  return {
    v: value,
    data: options?.data,
    sourceValue: options?.sourceValue ?? value,
    finaleValue: options?.finaleValue,
  };
}

export type NewOptions<T, S, D> = {
  finaleValue?: T,
  sourceValue?: S,
  data: D,
};

const instance_withError = make(0, {
   // Error here: `Type '{ test(): number; }' is not assignable to type 'undefined'.(2322)`?
    data: {
        test() {
            return 123;
        },
    },
});

const data = {
    test() {
        return 123;
    },
};
// no error here
const instance1 = make(0, {
    // data is not inline object, BUT object with non-arrow function
    data,
    sourceValue: 'test',
    finaleValue: 1,
});

// no error here
const instance2 = make(0, {
    // data is inline object, BUT object with arrow function
    data: {
         test: () => {
            return 123;
        },
    },
    sourceValue: 1,
    finaleValue: 2,
});
const sV1 = instance2.sourceValue;
//    ^?
const v1 = instance2.v;
//    ^?


const sV2 = instance1.sourceValue;
//    ^?
const v2 = instance1.v;
//    ^?


// Should be `{ test(): number; }`
const data_invalid = instance_withError.data;
//    ^?
// Should be `{ test(): number; }`
const data1 = instance1.data;
//    ^?
// Should be `{ test: () => number; }`
const data2 = instance2.data;
//    ^?

🙁 Actual behavior

An error occurs:
Type '{ test(): number; }' is not assignable to type 'undefined'.(2322)
data type is undefined

🙂 Expected behavior

No error occurs
data type is { test(): number; }

There's a lot of extra fluff in that repro, isn't it just this?

Playground

declare function make<D=undefined>(options: D): any;

// cant infer
make({
    test() {
        return 123;
    },
});

// can explictly call
make<{ test(): number }>({
    test() {
        return 123;
    },
})

// can infer with arrow function
make({
    test: () => {
        return 123;
    },
});

Likely a duplicate of #47599.

There's a lot of extra fluff in that repro, isn't it just this?

The reason why my example has 3 generic parameters

declare function make<T, S=T, D=undefined>(value: T, options: NewOptions<T, S, D>);

is to show that D=undefined default value is necessary here and cannot be removed.

Is you case

declare function make<D=undefined>(options: D): any;

we can just remove =undefined and all will work fine.

Likely a duplicate of #47599.

It's related, but not duplicate.
In my case, adding =undefined in generic parameter causes a problem.

This is already fixed by #62243 . You can try out nightly build.

Confirmed in nightly playground

I don't consider it is fixed. Exact situation here, errors still. Tried both on TS 6.0.3 and latest TSGO.

Simplified package code:

// ... (A ton of generics gymnastics)

type Context<
	M extends ReadonlyArray<ModuleConstructor<General>>,
	K extends Keys<InstanceEach<M>[number]>,
	Pr extends object = {},
> = MergeResult<M, K, Pr> & {
	__assign__: (obj: Partial<MergeResult<M, K, Pr>>) => Context<M, K, Pr>;
};

declare function createContext<
	M extends ReadonlyArray<ModuleConstructor<Context<M, K, Pr>>>,
	K extends Keys<InstanceEach<M>[number]>,
	Pr extends object = {},
>(
	classes: M,
	options: {
		preMerge?: Pr;
		mergeKeys: ReadonlyArray<K>;
	},
): Context<M, K, Pr>;

Test code (simplified):

// Works (arrow function)
const context = createContext(allModules, {
	mergeKeys: ['settings', 'root', 'events', 'i18n'],
	preMerge: {
		registerEvent: (ref: EventRef) => this.registerEvent(ref),
	},
}).__assign__({ settings: this.settings });

// Works (hoisting)
const preMerge = {
	registerEvent: this.registerEvent.bind(this),
};
const context = createContext(allModules, {
	mergeKeys: ['settings', 'root', 'events', 'i18n'],
	preMerge,
}).__assign__({ settings: this.settings });

// Fails, failure message shows that `Pr` is degraded to `{}` instead of exact `preMerge` type
const context = createContext(allModules, {
	mergeKeys: ['settings', 'root', 'events', 'i18n'],
	preMerge: {
		registerEvent: this.registerEvent.bind(this),
	},
}).__assign__({ settings: this.settings });

Since this is a closed issue, do I need to open a new one, or report in #47599, or do nothing?

Hēsperus (@hesprs)

Hēsperus (@hesprs) please log a new issue, thanks

Navigation Menu

Repository navigation

Bug with type argument inference if it has default value `= undefined` and actual value is object with non-arrow function #62995

🔎 Search Terms

🕗 Version & Regression Information

⏯ Playground Link

💻 Code

🙁 Actual behavior

🙂 Expected behavior

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Bug with type argument inference if it has default value = undefined and actual value is object with non-arrow function #62995

Description

🔎 Search Terms

🕗 Version & Regression Information

⏯ Playground Link

💻 Code

🙁 Actual behavior

🙂 Expected behavior

Activity

nmain commented on Jan 16, 2026

MartinJohns commented on Jan 16, 2026

termi commented on Jan 16, 2026

termi commented on Jan 16, 2026

Andarist commented on Jan 16, 2026

RyanCavanaugh commented on Jan 16, 2026

hesprs commented on Jun 22, 2026

RyanCavanaugh commented on Jun 24, 2026

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions

Bug with type argument inference if it has default value `= undefined` and actual value is object with non-arrow function #62995