microsoft / microsoft/TypeScript
Error: Debug Failure. No error for last overload signature
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Description
### 🔎 Search Terms
The TypeScript compiler crashs with the message "Error: Debug Failure. No error for last overload signature". (Check the DevTools console for the error message in the TypeScript Playground).
I've found issue #60229, which has the same error message. However, I am not sure if the root cause is the same.
### 🕗 Version & Regression Information
- The problem appears in the latest nightly version (v5.9.0-dev.20250403)
- This changed between versions 3.5.1 and 3.6.2 (in the Playground it works in 3.5.1 but not in 3.6.2 anymore)
### ⏯ Playground Link
https://www.typescriptlang.org/play/?ts=5.9.0-dev.20250403#code/C4TwDgpgBA4hB2EBOBLAxgHgCoD4oF4osoIAPYBAEwGcoBDeEAbQF0oB+Ip+AVwFsARsjYAuLgGsIIAPYAzIiwDcAKGVkw0pMCiye8NMBTT4UCtWAAxeBgCCJclVoAlCGk2UM51PADmAGnpGHBwACmUoKGkBACsxGz9wqDQBMRCU2ARkdAwABTotFDoAG1tggEoCPAYQZQqAb0TZTSgQt3hzKCZJEACAN2K2OSgAeRjXYAA6BGBUCGoQqOiy+sSI5JD+ovpaOERUTBscMpUIgF9lc6A
### 💻 Code
```ts
type Generic = T extends any[] ? T[number] : T[keyof T];
export function testFn>(
obj: A,
cb: (b: Generic>) => any
) {
for (const [key, val] of Object.entries(obj)) {
cb(val as Generic);
}
}
```
### 🙁 Actual behavior
The compiler crashs with:
```
Error: Debug Failure. No error for last overload signature
at resolveCall (M:\websites\app\node_modules\typescript\lib\_tsc.js:75576:19)
at resolveCallExpression (M:\websites\app\node_modules\typescript\lib\_tsc.js:75970:12)
at resolveSignature (M:\websites\app\node_modules\typescript\lib\_tsc.js:76398:16)
at getResolvedSignature (M:\websites\app\node_modules\typescript\lib\_tsc.js:76425:20)
at checkCallExpression (M:\websites\app\node_modules\typescript\lib\_tsc.js:76533:23)
at checkExpressionWorker (M:\websites\app\node_modules\typescript\lib\_tsc.js:79950:16)
at checkExpression (M:\websites\app\node_modules\typescript\lib\_tsc.js:79849:32)
at checkExpressionCached (M:\websites\app\node_modules\typescript\lib\_tsc.js:79473:28)
at checkVariableLikeDeclaration (M:\websites\app\node_modules\typescript\lib\_tsc.js:82311:35)
at checkVariableDeclaration (M:\websites\app\node_modules\typescript\lib\_tsc.js:82407:5)
```
### 🙂 Expected behavior
That the compiler does not crash.
### Additional information about the issue
Changing
```
cb: (b: Generic>) => any
```
to
```
cb: (b: Partial>) => any
```
prevents the error. However, this only addresses the symptoms rather than the root cause.
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 crash in the TypeScript Playground using the linked nightly version and the provided generic callback example. Start at the resolveCall and resolveCallExpression entries shown in the stack trace, compare behavior with versions 3.5.1 and 3.6.2, and verify that compiling the example no longer crashes.
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
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 45/100