Fall-through type narrowing

[moneppo]

I opened #22496 as I was using type checks in conditionals, and I realized V doesn’t keep narrowed types outside of the scope of the associated if/match. For instance:

type Foo = int | []int 
f := Foo{4}
if f is int {
  return false
}

// f must be []int, but this will fail compilation
return f[0]

I’m not sure if this is a feature you all thought about for V. It’s a feature of Typescript - were you considering this for V too?