[Pitch] Property-Based Type Narrowing and Union Types

[YOCKOW]

I think "Union Types" is listed in Commonly Rejected Changes.

• Disjunctions (logical ORs) in type constraints: These include anonymous union-like types (e.g. (Int | String) for a type that can be inhabited by either an integer or a string). "[This type of constraint is] something that the type system cannot and should not support."

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@amyworrall

Would this now be valid?

if foo != nil {
  foo.doSomething()
}

That is source breaking change:

@ensan-hcl

About if value != nil, it is really problematic. Currently this code works.

var value: Int? = 42
// some process
if value != nil {
    // some process     
    // finally make value nil
    value = nil
}

If Swift introduces if value != nil that implicitly unwraps value, it starts causing error, because value inside if is no longer Int? but Int. It's source breaking change actually.