[zig cc] command line -isystem paths are incorrectly searched after zig's system paths, instead of before #24243
Description
Zig Version
0.15-dev
Steps to Reproduce and Observed Behavior
I figured out why it happens. zig cc is not behaving like gcc and clang do with -isystem and -I directories.
zig cc
[m@M-TH build]$ zig cc -E -H iconv.c -isystem /home/m/static-php-cli/buildroot/include | grep iconv
. /usr/lib/zig/libc/include/generic-glibc/iconv.h
.. /usr/lib/zig/libc/include/generic-glibc/features.h
... /usr/lib/zig/libc/include/generic-glibc/features-time64.h
.... /usr/lib/zig/libc/include/x86_64-linux-gnu/bits/wordsize.h
.... /usr/lib/zig/libc/include/x86_64-linux-gnu/bits/timesize.h
..... /usr/lib/zig/libc/include/x86_64-linux-gnu/bits/wordsize.h
... /usr/lib/zig/libc/include/generic-glibc/stdc-predef.h
... /usr/lib/zig/libc/include/generic-glibc/sys/cdefs.h
.... /usr/lib/zig/libc/include/x86_64-linux-gnu/bits/wordsize.h
.... /usr/lib/zig/libc/include/x86_64-linux-gnu/bits/long-double.h
... /usr/lib/zig/libc/include/x86_64-linux-gnu/gnu/stubs.h
.... /usr/lib/zig/libc/include/x86_64-linux-gnu/gnu/stubs-64.h
...clang:
[m@M-TH build]$ clang -E -H iconv.c -isystem /home/m/static-php-cli/buildroot/include | grep iconv
. /home/m/static-php-cli/buildroot/include/iconv.h
.. /usr/bin/../lib/clang/19/include/stddef.h
... /usr/bin/../lib/clang/19/include/__stddef_header_macro.h
... /usr/bin/../lib/clang/19/include/__stddef_ptrdiff_t.h
... /usr/bin/../lib/clang/19/include/__stddef_size_t.h
... /usr/bin/../lib/clang/19/include/__stddef_wchar_t.h
... /usr/bin/../lib/clang/19/include/__stddef_null.h
... /usr/bin/../lib/clang/19/include/__stddef_max_align_t.h
... /usr/bin/../lib/clang/19/include/__stddef_offsetof.h
...gcc:
[m@M-TH build]$ gcc -E -H iconv.c -isystem /home/m/static-php-cli/buildroot/include | grep iconv
. /home/m/static-php-cli/buildroot/include/iconv.h
.. /usr/lib/gcc/x86_64-redhat-linux/14/include/stddef.h
.. /usr/include/errno.h
... /usr/include/features.h
.... /usr/include/features-time64.h
..... /usr/include/bits/wordsize.h
..... /usr/include/bits/timesize.h
...... /usr/include/bits/wordsize.h
.... /usr/include/sys/cdefs.h
..... /usr/include/bits/wordsize.h
..... /usr/include/bits/long-double.h
.... /usr/include/gnu/stubs.h
..... /usr/include/gnu/stubs-64.h
...But the real issue is this:
[m@M-TH build]$ zig cc -E -H iconv.c -isystem /home/m/static-php-cli/buildroot/include -I/home/m/static-php-cli/buildroot/include | grep iconv
. /usr/lib/zig/libc/include/generic-glibc/iconv.h
.. /usr/lib/zig/libc/include/generic-glibc/features.h
... /usr/lib/zig/libc/include/generic-glibc/features-time64.h
.... /usr/lib/zig/libc/include/x86_64-linux-gnu/bits/wordsize.h
.... /usr/lib/zig/libc/include/x86_64-linux-gnu/bits/timesize.h
..... /usr/lib/zig/libc/include/x86_64-linux-gnu/bits/wordsize.h
... /usr/lib/zig/libc/include/generic-glibc/stdc-predef.h
# or
[m@M-TH build]$ zig cc -E -H iconv.c -I/home/m/static-php-cli/buildroot/include -isystem /home/m/static-php-cli/buildroot/include | grep iconv
. /usr/lib/zig/libc/include/generic-glibc/iconv.h
.. /usr/lib/zig/libc/include/generic-glibc/features.h
... /usr/lib/zig/libc/include/generic-glibc/features-time64.h
.... /usr/lib/zig/libc/include/x86_64-linux-gnu/bits/wordsize.h
.... /usr/lib/zig/libc/include/x86_64-linux-gnu/bits/timesize.h
..... /usr/lib/zig/libc/include/x86_64-linux-gnu/bits/wordsize.h
... /usr/lib/zig/libc/include/generic-glibc/stdc-predef.hZig ignores the -I directory, if it is also specified as a -isystem dir.
zig cc without isystem:
[m@M-TH build]$ zig cc -E -H iconv.c -I/home/m/static-php-cli/buildroot/include | grep iconv
. /home/m/static-php-cli/buildroot/include/iconv.h
.. /usr/lib/zig/include/stddef.h
... /usr/lib/zig/include/__stddef_header_macro.h
... /usr/lib/zig/include/__stddef_ptrdiff_t.h
... /usr/lib/zig/include/__stddef_size_t.h
... /usr/lib/zig/include/__stddef_wchar_t.h
... /usr/lib/zig/include/__stddef_null.h
... /usr/lib/zig/include/__stddef_max_align_t.h
... /usr/lib/zig/include/__stddef_offsetof.h
.. /usr/lib/zig/libc/include/generic-glibc/errno.hSince the php build unconditionally adds all our directories as isystem directories to supress warnings during extension compilation, this cannot be worked around short of patching zig.