[MacPorts] #53758: gcc6 @6.3.0_2: g++ fails to find stdc++ includes when -isysroot set

[MacPorts]

#53758: gcc6 @6.3.0_2: g++ fails to find stdc++ includes when -isysroot set
----------------------+-------------------
 Reporter:  ibethune  |      Owner:
     Type:  defect    |     Status:  new
 Priority:  Normal    |  Milestone:
Component:  ports     |    Version:  2.4.0
 Keywords:            |       Port:  gcc6
----------------------+-------------------
 I just did a 'port upgrade outdated' which installed gcc6 @6.3.0_2
 (previously I had @6.2.0_2).  The following example is now failing for me
 since the upgrade:

 {{{
 Iains-MacBook-Pro:Desktop ibethune$ cat test.cpp
 #include <string>

 int main(int argc, char **argv) { return 0; }

 Iains-MacBook-Pro:Desktop ibethune$ g++ -v
 -isysroot=/Developer/SDKs/MacOSX10.9.sdk/ -o test test.cpp
 Using built-in specs.
 COLLECT_GCC=g++
 COLLECT_LTO_WRAPPER=/opt/local/libexec/gcc/x86_64-apple-darwin16/6.3.0
 /lto-wrapper
 Target: x86_64-apple-darwin16
 Configured with:
 /opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_lang_gcc6/gcc6/work/gcc-6.3.0/configure
 --prefix=/opt/local --build=x86_64-apple-darwin16 --enable-
 languages=c,c++,objc,obj-c++,lto,fortran --libdir=/opt/local/lib/gcc6
 --includedir=/opt/local/include/gcc6 --infodir=/opt/local/share/info
 --mandir=/opt/local/share/man --datarootdir=/opt/local/share/gcc-6 --with-
 local-prefix=/opt/local --with-system-zlib --disable-nls --program-
 suffix=-mp-6 --with-gxx-include-dir=/opt/local/include/gcc6/c++/ --with-
 gmp=/opt/local --with-mpfr=/opt/local --with-mpc=/opt/local --with-
 isl=/opt/local --enable-stage1-checking --disable-multilib --enable-lto
 --enable-libstdcxx-time --with-build-config=bootstrap-debug --with-
 as=/opt/local/bin/as --with-ld=/opt/local/bin/ld --with-
 ar=/opt/local/bin/ar --with-bugurl=https://trac.macports.org/newticket
 --with-pkgversion='MacPorts gcc6 6.3.0_2' --with-sysroot=
 Thread model: posix
 gcc version 6.3.0 (MacPorts gcc6 6.3.0_2)
 COLLECT_GCC_OPTIONS='-v' '-isysroot' '=/Developer/SDKs/MacOSX10.9.sdk/'
 '-o' 'test' '-mmacosx-version-min=10.12.4' '-asm_macosx_version_min=10.12'
 '-shared-libgcc' '-mtune=core2'
  /opt/local/libexec/gcc/x86_64-apple-darwin16/6.3.0/cc1plus -quiet -v
 -D__DYNAMIC__ -isysroot =/Developer/SDKs/MacOSX10.9.sdk/ test.cpp -fPIC
 -quiet -dumpbase test.cpp -mmacosx-version-min=10.12.4 -mtune=core2
 -auxbase test -version -o
 /var/folders/fk/ljxr9h0x2zx1lcjrtskgv02r0000gn/T//ccnNmN3P.s
 GNU C++14 (MacPorts gcc6 6.3.0_2) version 6.3.0 (x86_64-apple-darwin16)
         compiled by GNU C version 6.3.0, GMP version 6.1.2, MPFR version
 3.1.4, MPC version 1.0.3, isl version 0.15
 GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
 ignoring nonexistent directory
 "=/Developer/SDKs/MacOSX10.9.sdk/opt/local/include/gcc6/c++/"
 ignoring nonexistent directory
 "=/Developer/SDKs/MacOSX10.9.sdk/opt/local/include/gcc6/c++//x86_64-apple-
 darwin16"
 ignoring nonexistent directory
 "=/Developer/SDKs/MacOSX10.9.sdk/opt/local/include/gcc6/c++//backward"
 ignoring nonexistent directory
 "=/Developer/SDKs/MacOSX10.9.sdk/opt/local/include"
 ignoring nonexistent directory "/opt/local/lib/gcc6/gcc/x86_64-apple-
 darwin16/6.3.0/../../../../../x86_64-apple-darwin16/include"
 ignoring nonexistent directory
 "=/Developer/SDKs/MacOSX10.9.sdk/usr/include"
 ignoring nonexistent directory
 "=/Developer/SDKs/MacOSX10.9.sdk//System/Library/Frameworks"
 ignoring nonexistent directory
 "=/Developer/SDKs/MacOSX10.9.sdk//Library/Frameworks"
 #include "..." search starts here:
 #include <...> search starts here:
  /opt/local/lib/gcc6/gcc/x86_64-apple-darwin16/6.3.0/include
  /opt/local/lib/gcc6/gcc/x86_64-apple-darwin16/6.3.0/include-fixed
 End of search list.
 GNU C++14 (MacPorts gcc6 6.3.0_2) version 6.3.0 (x86_64-apple-darwin16)
         compiled by GNU C version 6.3.0, GMP version 6.1.2, MPFR version
 3.1.4, MPC version 1.0.3, isl version 0.15
 GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
 Compiler executable checksum: d36822ecd9de6fb879bb93e66b8d0005
 test.cpp:1:18: fatal error: string: No such file or directory
  #include <string>
                   ^
 compilation terminated.
 }}}

 With the previous versions (e.g. gcc5) it also searches under directory
 {{{/opt/local/include/gcc5/c++/}}}.  So how should I build using gcc6
 targetting an old macOS SDK?

--
Ticket URL: <https://trac.macports.org/ticket/53758>
MacPorts <https://www.macports.org/>
Ports system for macOS