The `base-devel` package group has recently been replaced by a meta package of the same name.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: https://archlinux.org/news/switch-to-the-base-devel-meta-package-requires-ma...