Error: Debug Failure. No error for last overload signature

[jens-duttke]

🔎 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

type Generic<T> = T extends any[] ? T[number] : T[keyof T];

export function testFn<A extends Record<string, any>>(
  obj: A,
  cb: (b: Generic<Partial<A>>) => any
) {
  for (const [key, val] of Object.entries(obj)) {
    cb(val as Generic<A>);
  }
}

🙁 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<Partial<A>>) => any

to

cb: (b: Partial<Generic<A>>) => any

prevents the error. However, this only addresses the symptoms rather than the root cause.

[Andarist]

At least part of the issue here is that relateVariances differs its behavior on reportErrors. First it fails and returns Ternary.False, exiting the relationship check early~ but then when the compiler looks for errors it returns undefined and continues with the check and it succeeds.

From this angle alone, https://github.com/microsoft/TypeScript/pull/55222 would fix this (I checked that)