TS4053 happens when public class method adopts imported type from library

[hkleungai]

Steps to reproduce

1. Start a plain new project
   a. "@typescript/native-preview": "^7.0.0-dev.20260225.1"
   b. "typescript": "6.0.0-beta"
2. Run npx tsgo --init to get the default tsconfig.
3. Add a src/lib.ts & src/main.ts. Run npx tsc --noEmit & npx tsgo --noEmit to see the difference.

// lib.ts
interface Base { }

export type ExportBase = Base;

// main.ts
import type { ExportBase } from './lib.js'

interface Clonable<T> {
    clone(): Clonable<T>
}

class Internal_Class {
    with_key_ExportBase_param(param: { key: ExportBase }) {
        if (Math.random() > 0.5) {
            param;
        } else {
            return param;
        }
    };

    with_clonable_key_ExportBase_param(param: Clonable<{ key: ExportBase }>) {
        if (Math.random() > 0.5) {
            param.clone();
        } else {
            return param.clone();
        }
    };

    with_ExportBase_param(param: ExportBase) {
        if (Math.random() > 0.5) {
            param;
        } else {
            return param;
        }
    };

    with_clonable_ExportBase_param(param: Clonable<ExportBase>) {
        if (Math.random() > 0.5) {
            param.clone();
        } else {
            return param.clone();
        }
    };
}

export default class Exported_Class {
    with_key_ExportBase_param(param: { key: ExportBase }) {
        if (Math.random() > 0.5) {
            param;
        } else {
            return param;
        }
    };

    with_clonable_key_ExportBase_param(param: Clonable<{ key: ExportBase }>) {
        if (Math.random() > 0.5) {
            param.clone();
        } else {
            return param.clone();
        }
    };

    with_ExportBase_param(param: ExportBase) {
        if (Math.random() > 0.5) {
            param;
        } else {
            return param;
        }
    };

    with_clonable_ExportBase_param(param: Clonable<ExportBase>) {
        if (Math.random() > 0.5) {
            param.clone();
        } else {
            return param.clone();
        }
    };
}

Behavior with typescript@6.0

src/main.ts:59:5 - error TS4053: Return type of public method from exported class has or is using name 'Base' from external module "project-directory/src/lib" but cannot be named.

59     with_ExportBase_param(param: ExportBase) {
       ~~~~~~~~~~~~~~~~~~~~~

src/main.ts:67:5 - error TS4053: Return type of public method from exported class has or is using name 'Base' from external module "project-directory/src/lib" but cannot be named.

67     with_clonable_ExportBase_param(param: Clonable<ExportBase>) {
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


Found 2 errors in the same file, starting at: src/main.ts:59

Behavior with tsgo

src/main.ts:43:5 - error TS4053: Return type of public method from exported class has or is using name 'Base' from external module "project-directory/src/lib" but cannot be named.

43     with_key_ExportBase_param(param: { key: ExportBase }) {
       ~~~~~~~~~~~~~~~~~~~~~~~~~

src/main.ts:51:5 - error TS4053: Return type of public method from exported class has or is using name 'Base' from external module "project-directory/src/lib" but cannot be named.

51     with_clonable_key_ExportBase_param(param: Clonable<{ key: ExportBase }>) {
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

src/main.ts:59:5 - error TS4053: Return type of public method from exported class has or is using name 'Base' from external module "project-directory/src/lib" but cannot be named.

59     with_ExportBase_param(param: ExportBase) {
       ~~~~~~~~~~~~~~~~~~~~~

src/main.ts:67:5 - error TS4053: Return type of public method from exported class has or is using name 'Base' from external module "project-directory/src/lib" but cannot be named.

67     with_clonable_ExportBase_param(param: Clonable<ExportBase>) {
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


Found 4 errors in the same file, starting at: src/main.ts:43

---

I did find myself a few workarounds.

1. Add back or remove all return on all the branches in method(); but this could be impractical for a large codebase.
2. Export Base together with ExportBase; but it requires (hacky) package patching when it comes to 3rd party dependencies..

Also note that the error goes away if ExportBase is NOT purely an alias of Base. In other words, the below typing will actually pass tsc & tsgo typechecking.

- export type ExportBase = Base;
+ export type ExportBase = Base & { extra: number; };

[hkleungai]

I made some edits on the issue description, after I twist my original code on further subcases.

• Exported vs Internal classes => Looks like internal class is just fine for both tsc & tsgo.
• With or without generic => No difference, seems.
• ExportBase vs { key: ExportBase }
  • The plain ExportBase is a full failure
  • The key one can be inferred correctly on ts6 but not ts7.

It looks like not even ts6 resolves all subcases as I intended. I wonder if I am hitting a known bug or limitation on tsc. But then, if tsc can yield some success, I do hope that it can be carried into ts7 too...

Hope this information can give more insight to the human / ai contributors :)
Let me know if I should open a new issue on ts6 repo.

[hkleungai]

Ohh, so someone had reported a very similar issue already, #2504.

Their cannot be named error happened on exported variable, whereas mine happens on public method from exported class. The TS error codes of the two are different, maybe this is why my previous PR search does not contain the issue...

But again, seems even ts6 is not handling all cases properly as expected. See my previous comment :)

[hkleungai]

Thanks for the hard work in #2459. With this PR takes effect on latest tsgo release, I believe a series of similar issues (e.g. 3027, 3094) has been resolved. 👍

But, for this issue 2911, there are still unresolved subcases that persist in both ts6 & ts7.

// lib.ts
interface Base { }

export type ExportBase = Base;

// failed.ts
import type { ExportBase } from './lib.js'

interface Clonable<T> {
    clone(): Clonable<T>
}

export default class Exported_Class {
    with_ExportBase_param(param: ExportBase) {
        if (Math.random() > 0.5) {
            param;
        } else {
            return param;
        }
    };

    with_clonable_ExportBase_param(param: Clonable<ExportBase>) {
        if (Math.random() > 0.5) {
            param.clone();
        } else {
            return param.clone();
        }
    };
}

Below are the errors from both typescript@6.0.1-rc & @typescript/native-preview@7.0.0-dev.20260321.1.

src/failed.ts:9:5 - error TS4053: Return type of public method from exported class has or is using name 'Base' from external module "project-directory/src/lib" but cannot be named.

9     with_ExportBase_param(param: ExportBase) {
      ~~~~~~~~~~~~~~~~~~~~~

src/failed.ts:17:5 - error TS4053: Return type of public method from exported class has or is using name 'Base' from external module "project-directory/src/lib" but cannot be named.

17     with_clonable_ExportBase_param(param: Clonable<ExportBase>) {
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


Found 2 errors in the same file, starting at: src/failed.ts:9

Ping @jakebailey since I see you assign yourself on this issue. :)

[jakebailey]

I'm only assigned because I assigned copilot to it, which didn't really work.

[jakebailey]

persist in both ts6 & ts7.

If so, then it's not a regression, no?

[hkleungai]

No, I suppose. Either a new bug, or a re-discovery on top of existing issues, on typescript in general.

If needed, I can open a new issue to typescript repo.