Access specifiers and visibility

[Andrew Edwards via Digitalmars-d-learn]

Attempting to update a git repo to current D, I encounter the 
following deprecation messages:

src/glwtf/signals.d-mixin-256(256,2): Deprecation: 
glwtf.input.BaseGLFWEventHandler._on_key_down is not visible from 
module glwtf.signals
src/glwtf/signals.d-mixin-256(256,2): Deprecation: 
glwtf.input.BaseGLFWEventHandler._on_key_up is not visible from 
module glwtf.signals
src/glwtf/signals.d-mixin-256(256,2): Deprecation: 
glwtf.input.BaseGLFWEventHandler._on_mouse_button_down is not 
visible from module glwtf.signals
src/glwtf/signals.d-mixin-256(256,2): Deprecation: 
glwtf.input.BaseGLFWEventHandler._on_mouse_button_up is not 
visible from module glwtf.signals

The offending line of code in signal.d for all four messages is:

        _impl.addSlot(obj, cast(void 
delegate())mixin("&obj."~method));

these methods are implemented in the following default 
constructor:

    class BaseGLFWEventHandler : AEventHandler {
        // [...]
        this() {
            on_key_down.connect!"_on_key_down"(this);
            on_key_up.connect!"_on_key_up"(this);
            
on_mouse_button_down.connect!"_on_mouse_button_down"(this);
            
on_mouse_button_up.connect!"_on_mouse_button_up"(this);
        }
        // [...]
    }

Which are implemented in the following abstract class:

    abstract class AEventHandler {
        // input
        Signal!(int, int, int) on_key_down;
        Signal!(int, int, int) on_key_up;
        Signal!(int, int) on_mouse_button_down;
        Signal!(int, int) on_mouse_button_up;
    }

I'm not sure how to address this issue so am seeking guidance on 
how to update this code such that it complies with current D.

Thanks,
Andrew