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.