Ian Stakenvicius wrote:
> Technically it could, but the issue here would be what you are going
> to do with a has_version check on an IUSE_RUNTIME dep -- the package
> should do filesystem-identical installs no matter what status of
> IUSE_RUNTIME flags, so whatever one would do with a has_version check
> would have to not change any part of the build or installation.

In principle it would be used for more or less the same thing as it 
would in a dependency, i.e. check whether the runtime-only dependencies 
for that feature are satisfied - the difference being that the package 
can specify arbitrary if-yes and if-no behaviours, rather than just 
"fail the dependency resolution" or not.  (Modulo the problem being 
discussed in this subthread, that a "no" answer isn't reliable.)

For example, some tool used during the build might have a "slow" mode 
that always works, and a "fast" mode that requires some other program to 
be installed and that has to be requested explicitly.  So the package 
that uses the tool might want to do something like

src_compile() {
     if has_version dev-util/buildtool[fast]; then
         buildtool --fast
     else
         buildtool
     fi
}