Not sure if something like this would be possible,...

# language-proposals

[ushort]

Not sure if something like this would be possible, but seems like possibly nice idea. Would let you only access foo within it's scope versus having to declare outside or reusing.

if (val foo = someClass.someProperty == 1) {
            // can access foo here
        }

[dmitriy.novozhilov]

Your syntax is very hard to read. Intuitively val foo = someClass.someProperty == 1 reads as val foo = (someClass.someProperty == 1), not (val foo = someClass.someProperty) == 1 (like val declaration in when)
If you really don't want to intoduce foo to outer scope you can use some scope function

someClass.someProperty.let { foo -> 
    if (foo == 1) { ... }
}

or just replace if with when

when (val foo = someClass.someProperty) {
    1 -> ...
}

[ushort]

Ah fair enough didn't think about it as assigning the condition, can see how that's easily misunderstood.