From Bulbanews, your community Pokémon newspaper.
Super Smash Bros. for Wii U will be playable with GameCube controllers
The GameCube Controller Adapter (For Wii U)
At the end of the official YouTube video for the Super Smash Bros. Invitational @ E3 2014 Nintendo has announced The GameCube Controller Adapter (For Wii U). It will be used Super Smash Bros. for Wii U to make GameCube controllers compatible with the game. The adapter will have four GameCube controller slots similar to the Nintendo GameCube and Wii consoles. It is currently unknown if this adapter will be compatible with any other software such as Virtual Console games.