@resultBuilder
struct Builder<T> {
    static func buildExpression(_ expression: T) -> [T] {
        [expression]
    }
    
    static func buildBlock(_ components: [T]...) -> [T] {
        Array(components.joined())
    }
}

typealias OutsideString = String
typealias OutsideStringBuilder = Builder<String>

protocol Buildable {
    typealias InsideString = String
    typealias InsideStringBuilder = Builder<String>
    
    // these crash:
    //@Builder<InsideString>
    //@InsideStringBuilder
    
    // these work fine:
    //@OutsideStringBuilder
    //@Builder<OutsideString>
    //@Builder<String>
    
    func build() -> [String]
}

struct Building: Buildable {
    // at this site they all work; just using this as an example
    @InsideStringBuilder
    func build() -> [String] {
        "foo"
        "bar"
    }
}