This is GDC (GNU D Compiler), the D frontend for GCC.

D is a general-purpose programming language with static typing,
systems-level access, and C-like syntax. With the D Programming
Language, write fast, read fast, and run fast.

This package provides GCC 8 with C, C++ and D. It doesn't conflict with
GCC 5 shipped by Slackware.

Since GCC is built several times to bootstrap the D frontend written in
D with an older C++-version, be sure you have about 9 GB of disk space
in your temporary directory.
