在省略和破坏子类型之后,无法构造泛型类型的对象

typescript版本:3.7.5

type A = { _id: string }
type B = A & { one: string, two: string }
type C = Omit<B, "_id">;


const valueWithoutId: C = {one: "value-1", two: "value-2"};

// This works just fine...
const valueWithId: B = {...valueWithoutId, _id: "new-id"};

console.log(valueWithId);


function thisBreaks_butWhy<T extends A, U = Omit<T, "_id">>(_valueWithoutId: U) {

    // The following line breaks... Why?
    const _valueWithId: T = {..._valueWithoutId, _id: "new-id"};
}

 TS2322: Type 'U & { _id: string; }' is not assignable to type 'T'.
  'U & { _id: string; }' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'A'

我试着更明确地说:

function thisBreaks_butWhy<T extends A>(_valueWithoutId: Omit<T, "_id">) {

    // The following line breaks... Why?
    const _valueWithId: T = {..._valueWithoutId, _id: "new-id"};
}