Discriminated Union only auto resolve when it's basic props, not within callback

[alexis-regnaud]

TypeScript Version: 4.1.2

Search Terms:
interdependent props, Union changed for an Intersection, Discriminated Union

Code

// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.

Expected behavior:
Since I use Discriminated Union I would like to have a Union within the callback as well.

Actual behavior:
Within the callback, I have an intersection instead of Union. But that work well with basic param (Exemple selected in the Playgound)

Playground Link:
Playground

Related Issues:
#40647, #12184

Seems like it's issue that has brought a bunch of times, but I'm wondering why we still need to do the

function isCase1(props: AllProps): props is Case1 {
  return !!props.aParamOfCase1;
}

Only for the callback function

[RyanCavanaugh]

This is a correct error; if this were allowed it'd be straightforward to inhabit an Option[] with an Option, which is unsound

[alexis-regnaud]

It's a SimpleSelected | MultipleSelected so a Option | Option[] then newOption which is of type Option should be accepted. I'm kind of confused...

[RyanCavanaugh]

// This call is legal, but crashes
aFunction({
  selected: [], onChange: (a: Option[]) => {
    console.log(a.indexOf(a[0]));
  }
});