Linker errors with Interfaces

[Andrej Mitrovic]

Excerpt from TDPL, p213-214:

interface Transmogrifier
{
    // client interface
    final void thereAndBack()
    {
        transmogrify();
        untransmogrify();
    }
    
    // implementation interface
private:
    void transmogrify();     
    void untransmogrify(); 
}

class CardboardBox : Transmogrifier
{
    override private void transmogrify()
    {
        // get in the box
    }
    
    override private void untransmogrify()
    {
        // get out of the box
    }
}

void play()
{
    writeln("just playing");
}

void aDayInLife(Transmogrifier device, string mood)
{
    if (mood == "play")
    {
        device.transmogrify();
        play();
        writeln(typeid(device));
        device.untransmogrify();
    }
    else if (mood == "experiment")
    {
        device.thereAndBack();
    }
}

import std.stdio;

void main()
{
    aDayInLife(new CardboardBox, "play");
}


Calling device.transmogrify() and device.untransmogrify() should not even be allowed to compile. I can comment these out, but the linker will still fail when there's a call to device.thereAndBack(); device is an object that inherits from interface Transmogrifier, and "thereAndBack()" is a public method of the interface, so I'm not sure whats wrong with that call.

I get these in both cases:

output\interface_test.obj(interface_test) 
 Error 42: Symbol Undefined _D14interface_test14Transmogrifier12transmogrifyMFZv
output\interface_test.obj(interface_test) 
 Error 42: Symbol Undefined _D14interface_test14Transmogrifier14untransmogrifyMFZv


One more thing, here's a class that inherits from the same interface, but tries to make the overriden methods non-private by mistake:
interface Transmogrifier
{

    // client interface
    final void thereAndBack()
    {
        transmogrify();
        untransmogrify();
    }
    
    // implementation interface
private:
    void transmogrify();       
    void untransmogrify();   
}

class CardboardBox : Transmogrifier
{
    override void transmogrify()
    {
        // get in the box
    }
    
    override void untransmogrify()
    {
        // get out of the box
    }
}

I get these errors:
interface_test.d(23): Error: function interface_test.CardboardBox.transmogrify does not override any function
interface_test.d(28): Error: function interface_test.CardboardBox.untransmogrify does not override any function

The book has nice plausible error message alternatives:
// Error: Cannot change protection of transmogrify from private to public
// Error: Cannot change protection of untransmogrify from private to public

I'd prefer if DMD put out those kinds of error messages in this case (or something similar).