Unify sum type unwrapping with Option unwrapping

[ntrel]

In git we have:

• if sum is T {/* sum is type T here */} which shadows sum in the block.
• if sum is T as var { which safely unwraps sum with the name var.

Outside if, sum is T produces a boolean. What if sum is T produced an optional everywhere? If sum doesn't hold a T then sum is T would equal none. Then we could use optional handling with sum types:

if val := sum is T {/* use val as T */}
val2 := sum is T or {...}
// use val2 as T

We could make if opt { valid with an optional value so that if sum is T { works naturally. That would be useful for shadow unwrapping an optional:

fn f(val ?Type) {
    if val {
        val.foo() // val has type `Type`
    }
    else {
        println('no value')
    }
}

Shadowing seems at least as useful for safe unwrapping of an optional variable as it is for sum type instance unwrapping.

opt or could also work for shadow unwrapping an optional variable:

val or {...}
// val here has type `Type`

All this unifies safe sum type unwrapping with optional handling in the language, for consistency.

[danieldaeschle]

if sum.field is T as sum_field { should work.

[ntrel]

@danieldaeschle: OK, thanks. I edited the summary in light of that.