Using Homebrew to upgrade GCC to trunk

To upgrade GCC to the latest trunk version using Homebrew, try the following command:

brew unlink gcc
brew install --fetch-HEAD --HEAD gcc

Note that now your default GCC will be the trunk version. If you wish to switch back to the latest stable version you had installed it will look like

brew switch gcc 8.3.0

Switching back to the latest trunk would be

brew switch gcc HEAD-<sha>

where <sha> is the abbreviated SHA of the trunk version you want. For me <sha> is 252768. If you are already on the HEAD/trunk gcc branch and you want to upgrade to the most recent HEAD/trunk, the following command should do the trick:

brew upgrade --fetch-HEAD gcc

Without --fetch-HEAD Homebrew won't go look for a newer version of HEAD/trunk.

You may want to delete some older HEAD/trunk versions. The best bet to do this is probably doing it manually. brew --cellar gcc will show you the top level gcc directory under which all the latest stable releases and HEAD/trunk builds are installed. Simply rm -rf HEAD-<sha> corresponding to the old HEAD/trunk installation you want to remove.

If you accidentally delete a version of gcc while it's still linked (you didn't run brew switch gcc <version>) you can run brew cleanup --prune-prefix to remove empty directories and dead symbolic links in /usr/local

If you want to install an older version of gcc try the following:

brew install gcc@<maj-rev>

Where <maj-rev> is 5, 6 or 7. (For older GCCs in the 4.x branch there may be a few available i.e. 4.9, 4.8 etc. Try brew search gcc.) Other major versions installed with the @<maj-rev> syntax are installed in parallel and have the -<maj-rev> suffix on all the installed binaries, so no brew switch gcc ... is required.