dev-libs/boost-1.51-r5 has a icu USE flag dependency: icu? ( >=dev-libs/icu-3.6:= ), so after installation it requires icu from slot 0/0, so it tries to downgrade dev-libs/icu to 49.1.2 (instead of 50.1-r2).
(In reply to Dmitriy Dyomin from comment #0) > dev-libs/boost-1.51-r5 has a icu USE flag dependency: icu? ( > >=dev-libs/icu-3.6:= ), so after installation it requires icu from slot 0/0, > so it tries to downgrade dev-libs/icu to 49.1.2 (instead of 50.1-r2). That makes no sense. We need the entire portage output; something else must be involved.