How does the compiler transform this result builder syntax?

Using Swift
1

I just read the result builder proposal and had a question about how if-else statements are transformed. I understand a variable vMerged is declared before the statement and is then initialised from within the if-else transform so it can be used as a partial result in the enclosing block. However, I came across a strange situation and I wasn't able to figure out how the transform would look.

typealias CSC = CustomStringConvertible
struct ConditionalString<T: CSC, F: CSC>: CSC {
    var condition: Condition
    var description: String { "would be self.condition.assocVal" }
    
    enum Condition {
        case isTrue(T)
        case isFalse(F)
    }
}

@resultBuilder
struct ConditionalStringBuilder {
    static func buildBlock<T: CSC>(_ component: T) -> T {
        return component
    }
    static func buildEither<A: CSC, B: CSC>(first component: A) -> ConditionalString<A, B> {
        return ConditionalString(condition: .isTrue(component))
    }
    static func buildEither<C: CSC, D: CSC>(second component: D) -> ConditionalString<C, D> {
        return ConditionalString(condition: .isFalse(component))
    }
}

@ConditionalStringBuilder
func test() -> some CSC {
    if Bool.random() {
        false
    } else {
        "asdf"
    }
}

let conditionalString = test()
print(type(of: conditionalString))  // Prints ConditionalString<Bool, String> to my surprise

I was expecting this to error. This is because I don't think it's possible for vMerged to be given a type. Based on the proposal, this is what this transform would look like:

let vMerged: PartialResult  // What is the type of PartialResult? It can't be (T || F). That's not valid
if Bool.random() {
  var firstVar = false
  var firstBlock = BuilderType.buildBlock(firstVar)
  vMerged = BuilderType.buildEither(first: firstBlock). // What is the type of `B` here?
} else {
  var elseVar = "asdf"
  var elseBlock = BuilderType.buildBlock(elseVar)
  vMerged = BuilderType.buildEither(second: elseBlock)  // What is the type of `C` here?
}
return vMerged

According to the compiler, each buildEither function could be returning a completely different type. So how is it inferring the type of vMerged? Additionally, does it know the type at compile time or at run time? If someone can please explain a step-by-step process of how this all works it'll really clear up a lot.

Thanks.