TS 7: declaration emit property order depends on which other files are in the program #64727

🔎 Search Terms

declaration emit property order, undefined property, nondeterministic d.ts, checkers

🕗 Version & Regression Information

• changed between 6.0.3 and 7.x
• still present on main at aa81492

💻 Code

// a.ts
export const a = Math.random() > 0.5 ? { user: 1 } : { status: "a" };

// b.ts
export const b = Math.random() > 0.5 ? { status: "no" } : { status: "ok", user: 2 };

tsc --strict --declaration --emitDeclarationOnly b.ts
tsc --strict --declaration --emitDeclarationOnly a.ts b.ts

🙁 Actual behavior

b.d.ts changes depending on whether a.ts is in the program:

{ status: string; user?: undefined; }   // b.ts alone
{ user?: undefined; status: string; }   // a.ts + b.ts

It also changes with --checkers, since that decides which files share a checker.

🙂 Expected behavior

Same b.d.ts either way. 6.0.3 prints { status: string; user?: undefined; } in both cases.

⏯ Additional information about the issue

getUndefinedProperty caches the synthetic user?: undefined symbol by property name, so whichever object literal widens user first on that checker wins, and its declaration position decides where user prints in later, unrelated types. Keying the cache by the source property symbol fixes it. Happy to send the PR.