Partial<generic> fails at matching types

[testerez]

TypeScript Version: 2.4.0
Code

interface Foo { foo: string };

function test<T extends Foo>(): Partial<T> {
  return { foo: '' };
}

Expected behavior: should be ok

Actual behavior: [ts] Type '{ foo: ""; }' is not assignable to type 'Partial<T>'.

[RyanCavanaugh]

This is a correct error. Your definition for test says it's legal to write this:

test<{foo: "bar" | "qua"}>();

but your implementation clearly fails to return a Partial<{foo: "bar" | "qua"}>

[testerez]

Humm ok I see. Is there a way to work around this that you can think of?
Because in my implementation I d'ont expect that a subtype of string will be used...

[RyanCavanaugh]

What's T even doing there? Just write function test(): Partial<Foo>

[testerez]

Yes, I guess I simplified my example too much. Here is an example closer to what I'm trying to achieve:

interface Foo {
  foo: boolean;
}

const assign = <T>(o: T, newValues: Partial<T>): T => (
  Object.assign({}, o, newValues)
);

const toggleFoo = <T extends Foo>(o: T): T => assign(o, { foo: !o.foo });
// [ts] Argument of type '{ foo: boolean; }' is not assignable to parameter of type 'Partial<T>'.

const bar = {
  foo: false,
  bar: false,
}

const bar2 = toggleFoo(bar); // bar and bar2 have the same type

console.log(bar2); // ​​​​​{ foo: true, bar: false }​​​​​

[RyanCavanaugh]

You're going to need a type assertion here:

const toggleFoo = <T extends Foo>(o: T): T => assign(o, <T>{ foo: !o.foo });
                                                        ~~~

[testerez]

Ok thanks for the help

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.