Andrei reports:

This works with -allinst but not without:

import std.typecons;

interface IDrawable
{
    void drawLine(int x1, int y1, int x2, int y2);
}

class ImageDraw  // note: it does not inherit IDrawable.
{
    void drawLine(int x1, int y1, int x2, int y2) { }
}

/** Draw a rectangle outline. */
void drawRect(IDrawable draw)
{
    draw.drawLine(  0,   0, 100,   0);
    draw.drawLine(100,   0, 100, 100);
    draw.drawLine(  0, 100, 100, 100);
    draw.drawLine(  0,   0,   0, 100);
}

void main()
{
    auto imageDraw = new ImageDraw();
    //drawRect(imageDraw);  // error: can't call this, ImageDraw is not an IDrawable.

    // perform a structural cast.
    IDrawable i = wrap!IDrawable(imageDraw);
    drawRect(i);  // and now imageDraw can act as an IDrawable.
}