What's your use case @SPFabGerman ?

The widget is meant to be instantiated, even in declarative structure. Just like awful.titlebar.widget. See examples https://awesomewm.org/apidoc/popups_and_bars/awful.titlebar.html

It doesn't make sense to me to use the clienticon widget without specifying the client it should source the icon from.

Maybe an evolution could be to allow declarative usage with the named parameters pattern. But that's really just a stylistic improvement.

My use case is as a template for both a tasklist and a taglist:
(The taglist code is pretty similar.)

    local function update_tasklist_callback(self, c, index, objects)
        self:get_children_by_id("client_icon_role")[1].client = c
    end

    local tasklist = awful.widget.tasklist {
        screen  = s,
		...
        widget_template = {
            id     = "background_role",
            widget = wibox.container.background,
            create_callback = update_tasklist_callback,
            update_callback = update_tasklist_callback,
            {
                widget = wibox.container.place,
                {
                    layout = wibox.layout.fixed.horizontal,
                    {
                        widget  = wibox.container.margin,
                        margins = 2,
                        {
                            id     = "client_icon_role",
                            widget = awful.widget.clienticon,
                        },
                    },
                    {
                        id     = "text_role",
                        widget = wibox.widget.textbox,
                    }
                }
            }
        }
    }

Because I use it in a template I can only specify the client with the two callbacks. But these seem to only be called after the widget has already been created. I know that I can use an imagebox widget with the icon_role id to solve this, but that doesn't have as high of a resolution.

The code has been adapted from https://awesomewm.org/apidoc/widgets/awful.widget.tasklist.html where pretty much the same thing is done.

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.42%. Comparing base (9475bb9) to head (4ac7bb6).
⚠️ Report is 2 commits behind head on master.

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+ Hits        54457    54463    +6     
+ Misses       5269     5263    -6     
  Partials      506      506           

The default_template from awful.widget.tasklist does use the awful.widget.clienticon widget. So there is definitely something to dig.

Oops, forget to approve with my comment.