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

Open the contributing guide

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

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