Conversion of Partial<T> to T loses "optional"ness of properties

[orgads]

TypeScript Version: 4.1.0-dev.20200824

Search Terms: Partial optional

Code

function partialToFull<T>(x: Partial<T>): T { return x as T; }

interface Foo {
  bar?: string;
};

const foo: Foo = { bar: 'baz' };
const full = partialToFull(foo);
delete full.bar; // error TS2790: The operand of a 'delete' operator must be optional.

Expected behavior:
It should work, as the property is optional.

Actual behavior:
It fails.

If I explicitly assign the type to full, it works:

const full: Foo = partialToFull(foo);

Playground Link: Link

Related Issues: #13783

[RyanCavanaugh]

I'm not sure why this is the "expected" behavior -- the entire point of the partialToFull type signature is to erase optionality; that's what it's written to do.

[orgads]

Why? It returns T, and the type I sent it is Foo.

My use-case is deepmerge, which accepts 2 objects (possibly of the same type), and returns a merged object. I'd expect it to maintain optionality.

[RyanCavanaugh]

It accepts a Partial<T> and returns a T.

You gave it a Partial<{ bar: string }> and it returned a { bar: string }.

That's what inference is supposed to do.

[orgads]

I see. Thanks for the explanation.