TS2322: Type is assignable to constraint of type V, but V could be instantiated with a different subtype of constraint

Question

I have the following typescript function

export const f1 = <V extends Record<string, any> & { name: string }>(vars: Omit<V, 'name'>, name: string): V => {
    const v: V = {...vars, name}
    return v
};

which gives the error

TS2322: Type  Omit<V, "name"> & { name: string; }  is not assignable to type  V 
 Omit<V, "name"> & { name: string; }  is assignable to the constraint of type  V ,
  but  V  could be instantiated with a different subtype of constraint  Record<string, any> & { name: string; }

I do not see how that can be the case here.

Answer 1

It's not that intuitive but I think easy way to look at it is that the function should also work if you are to specify the generic explicitly f1<{}>(...).

So if you have type T1 that fulfils extends Record<string, any> & { name: string }

type T1 = {
  name: number;
  other: "stuff";
} & {
  name: string;
};

This more less resolves as

{
    name: never;
    other: "stuff";
}

Then Omit<T1, "name"> type is

{
    other: "stuff";
}

If you call f1 like

f1<T1>({ other: "stuff" }, "name");

Then when you have { ...vars, name} you get

{
    other: "stuff";
    name: string;
}

which does match specified generic

{
    name: never;
    other: "stuff";
}

Same example starting with a different type like

type T1 = {
  name: "some specific string";
  other: "stuff";
} & {
  name: string;
};

shows the same thing but maybe example is more realistic.