The base-devel package group has recently been replaced by a meta package of the same name.
People that had the base-devel package group installed (meaning people that installed base-devel before February 2nd) have to explicitly re-install it to get the new base-devel package installed on their system:
pacman -Syu base-devel
URL: Arch Linux - News: Switch to the base-devel meta package requires manual intervention