Crash (Debug Failure) in Nightly when checking unreachable code in highly malformed source

[na7ure-a]

🔎 Search Terms

Debug Failure. Expected -1 >= 0

assertDiagnosticLocation

checkSourceElementUnreachable

compiler crash nightly

🕗 Version & Regression Information

Observed in: TypeScript Nightly
Does not occur in: 5.7.3, 5.8.3, 5.9.3

⏯ Playground Link

(https://www.typescriptlang.org/zh/play/?allowJs=true&target=7&outFile=true&ts=6.0.0-dev.20251219#code/KYDwDg9gTgLgBAQwM4E8B2BjOAzArpmASwjUSigRQDEoIBbAQVUwB4AVAPgApl0MBJGMAoAjADbAAhAC44TPoOEJxwdhwCUsgAq06hJKrYBtALoc4AbwCwAKDj24GEkngRc8ALxxTAblsOcaEQAdwRCeC4nNBc4ADc4CGxEZgEhUQl1S38AhzcYADowXCQACx5Q8Lj1PzsHAF9s+yhgGFwoUjyauqA)

💻 Code

export async function arrayFromAsync<T>(asyncIterable!: AsyncIterable<T>): Promise<T[]> {
    const out = [];
    for await (const v of asyncIterable) {
        out.push(await v);
    }
    return out;
}

🙁 Actual behavior

.\ts-versions\nightly\node_modules.bin\tsc.cmd --ignoreConfig .\test-async-array.ts
D:\do\tscFuzz\ts-versions\nightly\node_modules\typescript\lib_tsc.js:123191
throw e;
^

Error: Debug Failure. Expected -2 >= 0
at assertDiagnosticLocation (D:\do\tscFuzz\ts-versions\nightly\node_modules\typescript\lib_tsc.js:13878:9)
at createFileDiagnostic (D:\do\tscFuzz\ts-versions\nightly\node_modules\typescript\lib_tsc.js:17717:3)
at checkSourceElementUnreachable (D:\do\tscFuzz\ts-versions\nightly\node_modules\typescript\lib_tsc.js:86649:74)
at checkSourceElementWorker (D:\do\tscFuzz\ts-versions\nightly\node_modules\typescript\lib_tsc.js:86427:11)
at checkSourceElement (D:\do\tscFuzz\ts-versions\nightly\node_modules\typescript\lib_tsc.js:86396:7)
at forEach (D:\do\tscFuzz\ts-versions\nightly\node_modules\typescript\lib_tsc.js:32:22)
at checkSourceFileWorker (D:\do\tscFuzz\ts-versions\nightly\node_modules\typescript\lib_tsc.js:86866:7)
at checkSourceFile (D:\do\tscFuzz\ts-versions\nightly\node_modules\typescript\lib_tsc.js:86825:69)
at checkSourceFileWithEagerDiagnostics (D:\do\tscFuzz\ts-versions\nightly\node_modules\typescript\lib_tsc.js:86952:5)
at D:\do\tscFuzz\ts-versions\nightly\node_modules\typescript\lib_tsc.js:86974:46

Node.js v22.19.0

🙂 Expected behavior

The compiler should emit standard syntax errors (e.g., TS1005: ',' expected) and terminate gracefully without a "Debug Failure" crash, as it does in version 5.9.3.

Additional information about the issue

No response