Reverse mapped type fails to be inferred with constraint that is a union containing an intersection

🔎 Search Terms

reverse mapped type inference inferred constraint union intersection

🕗 Version & Regression Information

• This is the behavior in every version I tried. However, I'd call this to only be a bug for recent 5.4 nightlies - this should only work since https://github.com/microsoft/TypeScript/pull/55811

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.4.0-dev.20231231#code/CYUwxgNghgTiAEAzArgOzAFwJYHtVNQB4AVeEADwxFWAGd4AlcHGYQ2jGLVAcwBp4qZAFsARiBgA+SQAocogFYAueAG8AUPHgBtANLxu8GQGsQATxyJ4pAGTwARFHsBKeAB8Ho+wF0VxPd4A3OoAvs5+wWB4HPDyCvAAvGqa8FAqACwATHwpoioAjAAMhTlaYCqcyCA5IZHRGPBwtMgQDUmIqHKKzsEA9L1aWgB6APzqQA

💻 Code

declare function fn<T extends Record<string, number>>(obj: {
  [K in (keyof T & "a") | "b"]: T[K];
}): T;
const obj = {
  a: 42,
  b: 100,
  c: true,
};
const result = fn(obj);

🙁 Actual behavior

It fails to infer T and uses its constraint

🙂 Expected behavior

It should infer { a: number; b: number; }

Additional information about the issue

No response

Reproduce the behavior using the linked TypeScript Playground and the provided fn/object example. Trace why inference for T falls back to its constraint; the work is done when result is inferred as { a: number; b: number; } rather than the constraint, with a regression test covering the example.