microsoft / microsoft/TypeScript
TS2742 “The inferred type of 'default' cannot be named” with ESLint `defineConfig`
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Description
### 🔎 Search Terms
TS2742, 2742, composite, pnpm
### 🕗 Version & Regression Information
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about TS2742
### ⏯ Playground Link
_No response_
### 💻 Code
**`eslint.config.ts`**
```ts
import { defineConfig } from "@eslint/config-helpers";
export default defineConfig();
```
**`node_modules/@eslint/config-helpers/index.d.ts`**
```ts
export type Config = import("@eslint/core").ConfigObject;
export function defineConfig(): Config;
```
**`node_modules/@eslint/config-helpers/node_modules/@eslint/core/index.d.ts`**
```ts
export interface ConfigObject {}
```
**`tsconfig.json`**
```json
{
"compilerOptions": {
"composite": true
}
}
```
(No other files are needed for this test case; there’s nothing else in `node_modules`.)
### 🙁 Actual behavior
No error.
### 🙂 Expected behavior
`eslint.config.ts(2,1): error TS2742: The inferred type of 'default' cannot be named without a reference to '@eslint/config-helpers/node_modules/@eslint/core'. This is likely not portable. A type annotation is necessary.`
### Additional information about the issue
In eslint/rewrite#283 I reported an unexpected error TS2742 when using the `defineConfig` function from ESLint. I reduced this test case to a clear TypeScript issue, which I’m reporting as requested in https://github.com/microsoft/TypeScript/issues/42873#issuecomment-2065572017.
This change makes the error disappear:
```diff
--- a/node_modules/eslint/node_modules/@eslint/config-helpers/index.d.ts
+++ b/node_modules/eslint/node_modules/@eslint/config-helpers/index.d.ts
@@ -1,2 +1,3 @@
-export type Config = import("@eslint/core").ConfigObject;
+import { ConfigObject as Config } from "@eslint/core";
+export { Config };
export function defineConfig(): Config;
```
However, in the real `@eslint/config-helpers` package, the line `export type Config = import("@eslint/core").ConfigObject` is generated by `tsc` from JSDoc `/** @typedef {import("@eslint/core").ConfigObject} Config */`, and there doesn’t seem to be a way to generate an equivalent change from JSDoc.
In any case, such a change shouldn’t be necessary. TypeScript ought to recognize that `export type Config = import("@eslint/core").ConfigObject` is already sufficient to allow the `defineConfig` return type to be named.
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Reproduce the case from eslint.config.ts, node_modules/@eslint/config-helpers/index.d.ts, and tsconfig.json with composite enabled. Investigate how the compiler names the inferred default type when it uses the imported Config alias; done means the reduced example reports the expected TS2742 diagnostic without requiring a package change.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- typescript
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Quiet
- Clarity
- Clearly specified
- Newbie friendliness
- 48/100