According to the patent, which was unearthed by technology blog Siliconera, the physical buttons on the front of the case are created to interact with a touchscreen when the case is clasped over a smartphone.
Nintendo has filed a patent for a phone case that would turn your touch screen into a Game Boy with functioning buttons. The case has physical buttons and is created to look like an original Nintendo Game Boy system. The bonus is of course, you also get a case, so it's always on.
Apart from the dimensions, this case has a few differences from the original Game Boy. The smartphone would snap into place on the bottom layer of the case.
However, there's another aspect to the Switch's capacity to play games originally developed for other platforms, the ability to play mobile games on the big screen. The L and R buttons featured on the Game Boy Advance are not present. This would enable a Game Boy-esque system to be built on smart phones.
It looks like Nintendo is working on an official Game Boy case for smartphones. Nintendo has a huge backlog of wildly popular games, and to be able to jump into them on a bus for five minutes with a device I already own would be pretty fantastic. "Pokemon Go" was a global phenomenon when it launched in July 2016 and "Fire Emblem Heroes" has grossed more than $400 million worldwide since its launch in February 2017.
Whether there's enough good ones to justify what is probably going to be quite an expensive smartphone case remains to be seen though.