These are the packages that would be merged, in order:

Calculating dependencies ... done!
Dependency resolution took 0.98 s (backtrack: 0/20).


The following USE changes are necessary to proceed:
 (see "package.use" in the portage(5) man page for more details)
# required by net-misc/seafile-9.0.21::gentoo
# required by net-misc/seafile-client-9.0.21::gentoo
# required by =net-misc/seafile-client-9.0.21 (argument)
>=net-libs/libwebsockets-5.0.0-r1 socks5 client

!!! The ebuild selected to satisfy "net-libs/libwebsockets:=[client,socks5]" has unmet requirements.
- net-libs/libwebsockets-5.0.0-r1::gentoo USE="dbus ipv6 lejp smtp ssl threads -access-log -caps -cgi -client -extensions -generic-sessions -http-proxy -http2 -libev -libevent -libuv -mbedtls -peer-limits -server-status -socks5 -sqlite3 -zip"

  The following REQUIRED_USE flag constraints are unsatisfied:
    smtp? ( libuv ) socks5? ( http-proxy )

  The above constraints are a subset of the following complete expression:
    access-log? ( http2 ) generic-sessions? ( smtp sqlite3 ) http-proxy? ( client ) mbedtls? ( ssl ) smtp? ( libuv ) socks5? ( client http-proxy ) at-most-one-of ( libev libevent )

(dependency required by "net-misc/seafile-9.0.21::gentoo" [ebuild])
(dependency required by "net-misc/seafile-client-9.0.21::gentoo" [ebuild])
(dependency required by "=net-misc/seafile-client-9.0.21" [argument])