Criteria for picking compilers from fallback list ?

[Chris Jones]

Hi,

I'm working on an update to the root6 port, where the minimum compiler 
requirement has been increased in the latest release, such that I need 
to update the compiler blacklist/fallback to force a macports compiler 
to be used on OSX 10.8 and 10.9 (whereas previously the system one was OK).

This is all working OK, but I have noticed one thing I am trying to 
understand is on 10.8 and 10.9 different fallbacks are being picked.

I have

compiler.blacklist-append *gcc* {clang < 602} macports-clang-3.3 
macports-clang-3.4 macports-clang-3.5 macports-clang-3.6
compiler.fallback-append macports-clang-3.9 macports-clang-3.8 
macports-clang-3.7

On 10.8 this is causing macports clang 3.9 to be picked, whereas on 10.9 
clang 3.8 is being used. I am curious as to what the logic is in the 
blacklist portgroup that is leading to this different decisions to be 
made ? I thought perhaps it was based on what clang ports where already 
installed, but even after uninstalling all clang versions, the same 
decisions are being made. Just curious...

cheers Chris