The new shadow package is currently blocked by pam-login, as both provide /bin/login (the one from pam-login beeing deprecated).

[blocks B     ] sys-apps/pam-login (is blocking sys-apps/shadow-4.0.15-r2)

Another way of resolving the current upgrade conflict with pam-login and shadow on Gentoo (pam-login is now included in shadow, background on this posted here) without loosing /bin/login at any time during the upgrade process:

FEATURES=-collision-protect emerge --nodeps shadow && emerge --unmerge pam-login

Credits: Kerin M on gentoo-server mailing list