When compiling C++ code with Clang, libc++ can be used instead of libstdc++ as the standard C++ library by passing `-stdlib=libc++` on the compiler command line. If libc++ is not installed (package `sys-libs/libcxx`), the compiler switch is still available but linking or compiling will fail. In my opinion this behavior is undesirable because I expect libc++ to be available if this command line option exists. For me, the following behaviors are more desirable: * libc++ is always installed with Clang. * The command line option `-stdlib=libc++` is not available if libc++ was not installed. * The command line option `-stdlib=libc++` is always available but a clear error message is printed if libc++ is not installed. Clang version: ``` $ clang++ --version clang version 6.0.1 (tags/RELEASE_601/final) Target: x86_64-pc-linux-gnu Thread model: posix InstalledDir: /usr/lib/llvm/6/bin ``` Linker error: ``` $ cat main.cpp int main() {} $ clang++ -stdlib=libc++ main.cpp /usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: cannot find -lc++ clang-6.0: error: linker command failed with exit code 1 (use -v to see invocation) Compiler error with missing includes: ``` $ cat main.cpp #include <cstddef> int main() {} $ clang++ -stdlib=libc++ main.cpp main.cpp:1:10: fatal error: 'cstddef' file not found #include <cstddef> ^~~~~~~~~ 1 error generated. ``` $ emerge --info Portage 2.3.40 (python 2.7.15-final-0, default/linux/amd64/17.0/no-multilib, gcc-7.3.0, glibc-2.26-r7, 4.14.65-gentoo-a x86_64) ================================================================= System uname: Linux-4.14.65-gentoo-a-x86_64-AMD_Athlon-tm-_II_X2_270_Processor-with-gentoo-2.4.1 KiB Mem: 8163612 total, 2638812 free KiB Swap: 0 total, 0 free Timestamp of repository gentoo: Sat, 15 Sep 2018 11:45:01 +0000 Head commit of repository gentoo: 5d01953075fcfdb1fbb0e39c3f5f07271e4cf092 sh bash 4.4_p12 ld GNU ld (Gentoo 2.30 p2) 2.30.0 app-shells/bash: 4.4_p12::gentoo dev-lang/perl: 5.24.3-r1::gentoo dev-lang/python: 2.7.15::gentoo, 3.6.5::gentoo dev-util/cmake: 3.9.6::gentoo dev-util/pkgconfig: 0.29.2::gentoo sys-apps/baselayout: 2.4.1-r2::gentoo sys-apps/openrc: 0.34.11::gentoo sys-apps/sandbox: 2.13::gentoo sys-devel/autoconf: 2.13::gentoo, 2.69-r4::gentoo sys-devel/automake: 1.15.1-r2::gentoo sys-devel/binutils: 2.30-r2::gentoo sys-devel/gcc: 7.3.0-r3::gentoo sys-devel/gcc-config: 1.8-r1::gentoo sys-devel/libtool: 2.4.6-r3::gentoo sys-devel/make: 4.2.1-r4::gentoo sys-kernel/linux-headers: 4.13::gentoo (virtual/os-headers) sys-libs/glibc: 2.26-r7::gentoo Repositories: gentoo location: /usr/portage sync-type: rsync sync-uri: rsync://rsync.de.gentoo.org/gentoo-portage priority: -1000 sync-rsync-verify-jobs: 1 sync-rsync-verify-metamanifest: yes sync-rsync-extra-opts: sync-rsync-verify-max-age: 24 x-portage location: /usr/local/portage masters: gentoo priority: 0 ACCEPT_KEYWORDS="amd64" ACCEPT_LICENSE="* -@EULA" CBUILD="x86_64-pc-linux-gnu" CFLAGS="-O2 -pipe -march=native" CHOST="x86_64-pc-linux-gnu" CONFIG_PROTECT="/etc /usr/share/config /usr/share/gnupg/qualified.txt" CONFIG_PROTECT_MASK="/etc/ca-certificates.conf /etc/dconf /etc/env.d /etc/fonts/fonts.conf /etc/gconf /etc/gentoo-release /etc/revdep-rebuild /etc/sandbox.d /etc/terminfo /etc/texmf/language.dat.d /etc/texmf/language.def.d /etc/texmf/updmap.d /etc/texmf/web2c" CXXFLAGS="-O2 -pipe -march=native" DISTDIR="/usr/portage/distfiles" ENV_UNSET="DBUS_SESSION_BUS_ADDRESS DISPLAY PERL5LIB PERL5OPT PERLPREFIX PERL_CORE PERL_MB_OPT PERL_MM_OPT XAUTHORITY XDG_CACHE_HOME XDG_CONFIG_HOME XDG_DATA_HOME XDG_RUNTIME_DIR" FCFLAGS="-O2 -pipe" FEATURES="assume-digests binpkg-logs collision-protect config-protect-if-modified distlocks ebuild-locks fixlafiles merge-sync multilib-strict news parallel-fetch preserve-libs protect-owned sandbox sfperms strict unknown-features-warn unmerge-logs unmerge-orphans userfetch userpriv usersandbox usersync xattr" FFLAGS="-O2 -pipe" GENTOO_MIRRORS="http://ftp.halifax.rwth-aachen.de/gentoo/ http://ftp-stud.hs-esslingen.de/pub/Mirrors/gentoo/ http://mirror.eu.oneandone.net/linux/distributions/gentoo/gentoo/" LANG="en_US.utf8" LDFLAGS="-Wl,-O1 -Wl,--as-needed" MAKEOPTS="-j2" PKGDIR="/usr/portage/packages" PORTAGE_COMPRESS="" PORTAGE_CONFIGROOT="/" PORTAGE_RSYNC_OPTS="--recursive --links --safe-links --perms --times --omit-dir-times --compress --force --whole-file --delete --stats --human-readable --timeout=180 --exclude=/distfiles --exclude=/local --exclude=/packages --exclude=/.git" PORTAGE_TMPDIR="/var/tmp" USE="acl amd64 berkdb bindist bzip2 cli crypt cxx dri fortran gdbm iconv ipv6 jpeg libtirpc modules ncurses nls nptl openmp pam pcre png readline seccomp ssl tcpd unicode xattr zlib" ABI_X86="64" ALSA_CARDS="ali5451 als4000 atiixp atiixp-modem bt87x ca0106 cmipci emu10k1x ens1370 ens1371 es1938 es1968 fm801 hda-intel intel8x0 intel8x0m maestro3 trident usb-audio via82xx via82xx-modem ymfpci" APACHE2_MODULES="authn_core authz_core socache_shmcb unixd actions alias auth_basic authn_alias authn_anon authn_dbm authn_default authn_file authz_dbm authz_default authz_groupfile authz_host authz_owner authz_user autoindex cache cgi cgid dav dav_fs dav_lock deflate dir disk_cache env expires ext_filter file_cache filter headers include info log_config logio mem_cache mime mime_magic negotiation rewrite setenvif speling status unique_id userdir usertrack vhost_alias cgid proxy proxy_fcgi" CALLIGRA_FEATURES="karbon plan sheets stage words" COLLECTD_PLUGINS="df interface irq load memory rrdtool swap syslog" CPU_FLAGS_X86="3dnow 3dnowext mmx mmxext popcnt sse sse2 sse3 sse4a" ELIBC="glibc" GPSD_PROTOCOLS="ashtech aivdm earthmate evermore fv18 garmin garmintxt gpsclock isync itrax mtk3301 nmea ntrip navcom oceanserver oldstyle oncore rtcm104v2 rtcm104v3 sirf skytraq superstar2 timing tsip tripmate tnt ublox ubx" INPUT_DEVICES="evdev" KERNEL="linux" LCD_DEVICES="bayrad cfontz cfontz633 glk hd44780 lb216 lcdm001 mtxorb ncurses text" LIBREOFFICE_EXTENSIONS="presenter-console presenter-minimizer" OFFICE_IMPLEMENTATION="libreoffice" PHP_TARGETS="php5-6 php7-0" POSTGRES_TARGETS="postgres9_5 postgres10" PYTHON_SINGLE_TARGET="python3_6" PYTHON_TARGETS="python2_7 python3_6" RUBY_TARGETS="ruby23" USERLAND="GNU" VIDEO_CARDS="nouveau nvidia" XTABLES_ADDONS="quota2 psd pknock lscan length2 ipv4options ipset ipp2p iface geoip fuzzy condition tee tarpit sysrq steal rawnat logmark ipmark dhcpmac delude chaos account" Unset: CC, CPPFLAGS, CTARGET, CXX, EMERGE_DEFAULT_OPTS, INSTALL_MASK, LC_ALL, LINGUAS, PORTAGE_BINHOST, PORTAGE_BUNZIP2_COMMAND, PORTAGE_COMPRESS_FLAGS, PORTAGE_RSYNC_EXTRA_OPTS
It makes no sense to strip features and require complete rebuild of clang after installing libc++. The USE flags on clang-runtime give you control over which compiler runtime components are available. If you'd like clang to give better error message about missing libc++, I believe this is something to be reported upstream. However, I'm not sure if it's feasible for them to try to figure out if libc++ is installed. Writing a good patch might help.
