We knew the 4.0 Switch update added support for video capture and data transfers, but it seems Nintendo quietly snuck in support for GameCube controllers and didn't tell us.  Crafty!

Numerous people spotted the addition and GameXplain demonstrates this in the video below.  You need the Wii U GameCube Controller Adaptor that was released alongside Super Smash Bros. (although stock was limited), but as the video shows you can use it on the Switch.

NintendoWorldReport has also confirmed that WaveBird controllers also work.

As Eurogamer has reminded us, they did report nearly a year ago that Nintendo was looking into adding the GameCube Virtual Console to the Switch and GameCube controller support was considered as part of that.  Now that the controllers work, perhaps we'll finally see Virtual Console support on the Switch in the near future.