Cannot use type assertion to `Partial<T>` where T is generic

[DanielRosenwasser]

In #19467, @GregorioBR filed an issue about assigning a Foo to a Partial<T> where T extends Foo.

However, in trying to solve the issue with a type assertion, I got an error:

interface State {
    data?: string;
}

class View<S extends State> {

    public processState(): Partial<S> {
      return { data: "test" } as Partial<S>;
    }
}

This is surprising because I believe Partial<S> should be comparable to { data: "test" }.

[mhegazy]

Same issue referenced in

• Partial<T> assignability error where T extends a compatible type #19388
• Partial<this> not working as expected #16356
• Partial and Readonly are not commutative when using generics #17682
• Cannot define a Partial<T> for a constrained T extends Base specifying only members of Base #12799
• Partial does not work well with generic guards #18072
• T[K] not assignable to Partial<T[K]> #18201
• Type inference in generic return type #18839

[mhegazy]

@ahejlsberg we chatted about this issue earlier in the week. looking through the issues, there are multiple scenarios where this comes up, and our current behavior is not desired.

[DanielRosenwasser]

I don't think it's the same issue, but it's definitely related. This is about making it easier to say "I know what I'm doing".

[ahejlsberg]

@DanielRosenwasser Looking at this, the original example compiles with no errors with the current master (and, it appears, has since 2.5) because the comparable relation is more permissive that the assignable relation.

@mhegazy The issues you reference above all have to do with assignability. Are you suggesting we change the rules there? If so, what would the new rules be?

[mhegazy]

I originally wanted to discuss the all as a group, and we did. I have since closed all assignebilty issues as working as intended.
It should be fine to close this one as fixed now.