clang's driver ranks the host toolchain's own libc++ headers
(<install>/../include/c++/v1) above --sysroot. BuildStdPcm has forced
the sysroot's copy for the std module for that reason, but ordinary
compiles did not. A unit that only does `import std;` never noticed. A
unit that includes a standard header textually got the host's libc++,
configured for the host's libc: cross-building Crafter.Graphics for
aarch64 musl, Crafter.Asset's vendored gdeflate died on <chrono> with
"unknown rune table for this platform". Against a glibc sysroot the
same unit would compile against the wrong libc++ release and say
nothing.
Every non-wasm, non-mingw compile with a sysroot now gets
`-nostdinc++ -isystem <sysroot>/usr/include/c++/v1`. wasm has its own
sysroot handling and mingw is a libstdc++ target.
-nostdinc++ leaves -stdlib=libc++ unused on each compile, which clang
reports once per unit, so the command also carries
-Wno-unused-command-line-argument. The cost is that the driver's
unused-argument diagnostics are silent on sysroot builds, including for
a user's own misplaced flag (a -l or -Wl, option in compileFlags).
Not addressed: external cmake dependencies are still configured
without the target or the sysroot, and objects already built in an
existing sysroot build directory are not invalidated by the new flags.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Jorijn van der Graaf
3c96d78a7d