[gentoo-user] Please help me understand this emerge error message.

[Alan Mackenzie <acm@muc.de>]

Hello, Gentoo.

For the past couple of days, after $ emerge --sync, I've tried

    $ emerge -auND @world

..  It has come back very quickly with the error message:

#########################################################################

!!! The ebuild selected to satisfy ">=net-misc/curl-7.21.5[ssl]" has unmet requirements.
- net-misc/curl-8.1.2::gentoo USE="adns ftp gnutls http2 imap pop3 progress-meter smtp ssl tftp -alt-svc -brotli -gopher -hsts -idn -kerberos -ldap -mbedtls (-nghttp3) -nss -openssl -rtmp (-rustls) -samba -ssh (-sslv3) -static-libs -telnet
-test -verify-sig -websockets -zstd" ABI_X86="(64) -32 (-x32)" CURL_SSL="openssl -gnutls -mbedtls -nss (-rustls)"

  The following REQUIRED_USE flag constraints are unsatisfied:
    curl_ssl_openssl? ( openssl )

  The above constraints are a subset of the following complete expression:
    ssl? ( exactly-one-of ( curl_ssl_gnutls curl_ssl_mbedtls curl_ssl_nss curl_ssl_openssl curl_ssl_rustls ) ) curl_ssl_gnutls? ( gnutls ) curl_ssl_mbedtls? ( mbedtls ) curl_ssl_nss? ( nss ) curl_ssl_openssl? ( openssl ) curl_ssl_rustls? (
rustls )

#########################################################################

..  I don't understand what is meant by "curl_ssl_openssl? ( openssl )" in
the above message.  I assume it's talking about some USE flag, but which?
openssl?  curl_ssl_openssl?  As a matter of interest, I have
=dev-libs/openssl-3.0.9-r2 installed.

Please help me understand what's happening.

Thanks for the help!

-- 
Alan Mackenzie (Nuremberg, Germany).