Minecraft wiki entry for Development_versions, associated text: Mending is no longer compatible with Infinity on bows when trying to apply it in survival or when using the /enchant command.
In this snapshot, however, players can still add mending to an infinity bow, but not vice versa.
The player can also put Infinity on a Mending enchantment book, but not vice versa.
Old bows with both enchantments applied, will still keep their enchantments and remain working as they used to.