No idea if this can be fixed before Portage 2.1 goes stable, but it's worth tracking at least.
Not really sure what you mean with "slot safe" here, an example would probably help.
The sample I stumbled about was mythtv needing Qt 3 with mysql use flag enabled and having a relevant check. The point is having Qt 4 with mysql installed suffices to pass the check, but the ebuild will still fail. Looking at the function and its usage, it's also interesting to note that there're a lot of built_with_use <operator><category>/<package>-<version> calls, but the internally used best_version() does only take <category>/<package> as argument. At least that is how it is defined in portageq --help and e.g. portageq best_version / =x11-libs/qt-3 results in an empty string here with Portage 2.0.54-r2 Grepping the tree for this incorrect built_with_use() usage does not look good...
Eh, forget it. A typo in the depend atom, when I tried it. It's the mythtv ebuild that sucks.
