Given the following:

type X = {
  a: number;
  b: number;
};

const a = <T extends X>(t: T): T => {
  const { b, ...noB } = t;
  const r = { ...noB, b: 1 };
  return r;
};

Type of noB is Omit<T, "b">.

Type of r is Omit<T, "b"> & { b: number }

Typescript complains that

Type 'Omit<T, "b"> & { b: number; }' is not assignable to type 'T'.
  'Omit<T, "b"> & { b: number; }' is assignable to the constraint of type 'T', 
  but 'T' could be instantiated with a different subtype of constraint 'X'.ts(2322)

I don’t understand why there is a type mismatch here since it seems clear to me that Omit<T, "b"> & { b: number } will be always equivalent (assignable) to T.