1

Sample code:

public final class Box<T> where T: AnyObject {
    internal typealias Address = UInt

    @usableFromInline
    internal var address: Address

    @inlinable
    public init(object: T) {
        self.address = .init(bitPattern: Unmanaged<T>.passRetained(object).toOpaque())
    }
}

OS: macOS
Swift: Apple Swift version 4.2 (swiftlang-1000.0.16.7 clang-1000.10.25.3)
Clang: Apple LLVM version 10.0.0 (clang-1000.10.25.5)
Result: compile success.

OS: Ubuntu 14.14
Swift: version 4.2 (swift-4.2-CONVERGENCE)
Clang: clang version 5.0.0 (tags/RELEASE_500/final)
Result:

error: type alias 'Address' is internal and should not be referenced from an '@inlinable' function
note: type alias 'Address' is not '@usableFromInline' or public

@usableFromInline does not work on typealias.

2

@usableFromInline does work on typealiases now. You'll have to update to a later Xcode 10 beta, though.