Stack overflow narrowing `as const` object assigned to loop variable it references

[frankeld]

🔎 Search Terms

getTypeAtFlowLoopLabel, stack overflow loop narrowing, as const stack overflow, getAssignmentReducedType

🕗 Version & Regression Information

• This changed between versions 7.1.0-dev.20260723.1 and 7.1.0-dev.20260724.1
• 7.0.2 is fine; still crashes on 7.1.0-dev.20260825.1

⏯ Playground Link

No response

💻 Code

type Candidate = { mode: "a"; output: unknown } | { mode: "b" };

export function run(): never {
  let lastCandidate: Candidate | null = null;
  while (true) {
    const candidate: Candidate = {
      mode: "a",
      output: lastCandidate,
    } as const;
    lastCandidate = candidate;
  }
}

🙁 Actual behavior

tsc crashes every time:

runtime: goroutine stack exceeds 1000000000-byte limit
fatal error: stack overflow

getTypeAtFlowLoopLabel (flow.go:1325)
getTypeAtFlowNode (flow.go:174)
getTypeAtFlowCondition (flow.go:355)
getFlowTypeOfReferenceEx (flow.go:98)
checkIdentifier (checker.go:11267)
... ~3.2M frames elided

🙂 Expected behavior

Compiles clean (it does on 5.9.3 and on nightlies up to 20260723)

Additional information about the issue

All three parts are needed to crash:

• the annotation has to be a union (single object type is fine)
• the object has to be as const (removing it is the workaround)
• the object has to reference the loop variable it's assigned back to

[ahejlsberg]

This issue is caused by a change to checkAssertion made in microsoft/typescript-go#4708. #64013 works around the issue, but doesn't fix it. Same for #64018. It will put up a PR with the correct fix.