Setting up a new machine I've installed brew, before installing xcode. No big deal, brew downloaded something and was able to correctly install the packages I needed.
Then I manually installed other apps, included the latest version of xcode... and brew started to complain that I had an old xcode version installed and I needed to upgrade it through the mac appstore...
At the end, I had to use the command line utility xcode-select to select the correct xcode (the one I had download from the appstore) to convince brew that I had the latest version.
At least now I know!