Full-upgrade to Debian testing fails due to libc6-dev : Breaks: libgcc-8-dev

Installing gcc-8-base (sudo apt install gcc-8-base) appeared to do the trick for me and fix the problem for me.

If it still does not work try: apt-get install gcc-9-base

Clearly there are some broken dependencies.

Got the same issue.

# apt-get dist-upgrade
The following packages have unmet dependencies:
  libc6-dev : Breaks: libgcc-8-dev (< 8.4.0-2~) but 8.3.0-6 is to be installed
E: Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.

So, I ran #apt-get install libgcc-8-dev, hoping that it will upgrade the problematic package or get more information. The output was:

The following packages have unmet dependencies:
  gcc-8-base : Breaks: gcc-8-base:i386 (!= 8.4.0-4) but 8.3.0-6 is to be installed
  gcc-8-base:i386 : Breaks: gcc-8-base (!= 8.3.0-6) but 8.4.0-4 is to be installed
E: Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.

That's how I found that the gcc-8-base package is problematic. Next step? #apt-get install gcc-8-base gcc-8-base:i386. Result: gcc-8-base can be upgraded, but the apt want me to remove about 300 packages (which I don't want to remove, many of them are KDE-related) including gcc-8-base:i386, no alternative. I can work without gcc-8-base:i386, so removed it, but it didn't help.