The Portage cache

Portage uses a cache for most top-level variables (DEPEND, DESCRIPTION, SRC_URI and so on). This cache may be generated on a different machine, so these variables must be either static or generated using only unchanging 'version / name' variables (P, PN, PV, PR, PVR and PF).

The cache, when generated, must be identical independent of the used machine or environment. This concept is referred to as metadata invariance.

So, the following will not work:

# DO NOT DO THIS!
if ! has_version "x11-libs/gtk+" ; then
	DEPEND="${DEPEND}
		gtk?  ( >=x11-libs/gtk+-2 )
		!gtk? ( =x11-libs/gtk+-1.2* )"
fi

However, the following is legal, since the ver_test function works upon PV, and the PV and PN variables are both static:

if ver_test -ge 7.0 ; then
	IUSE="${IUSE} tcltk mzscheme"
	DEPEND="${DEPEND}
		tcltk?    ( dev-lang/tcl )
		mzscheme? ( dev-lisp/mzscheme )"
	RDEPEND="${RDEPEND}
		tcltk?    ( dev-lang/tcl )
		mzscheme? ( dev-lisp/mzscheme )"

	if [[ "${MY_PN}" != "vim-core" ]] ; then
		RDEPEND="${RDEPEND} !<app-vim/align-30-r1"
	fi
fi

Conditional inherits

Because eclasses modify various cached variables, conditional inheritance is not allowed except where the same results will always be obtained on every system. For example, inherits based upon USE flags are illegal, but inherits based solely upon the unchanging variables (e.g. PV) mentioned above are allowed.

As an example of a legal and possibly useful conditional inherit, some eclasses or ebuilds do:

if [[ ${PV} == 9999 ]]; then
	inherit git-r3
	EGIT_REPO_URI="https://anongit.gentoo.org/git/proj/devmanual.git"
else
	SRC_URI="https://dev.gentoo.org/~ulm/distfiles/${P}.tar.xz"
	KEYWORDS="~amd64 ~arm ~hppa ~ppc ~ppc64 ~sparc ~x86"
fi

This allows the same eclass (or the same ebuild "template") to be used for both regular and live packages.