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GCC 6 symbol poisoning and c++ header usage is fragile

building gcc6 using musl based gcc6 fails with symbol poisoning error
(details at the end of the mail).

the root cause is c++: c++ headers include random libc headers with
_GNU_SOURCE ftm so all sorts of unexpected symbols are defined/declared.

since it's unlikely the c++ standard gets fixed (to properly specify
the namespace rules) it is not acceptable to include std headers after
system.h, where the poisoning happens, because trivial libc header
change will break the build.

c++ header use in gcc seems inconsistent, e.g. there are cases where
- <string> is included before system.h (go-system.h)
- <string> is included after system.h (indirectly through <map>)
- <string> is included in system.h because INCLUDE_STRING is defined.
- <new> is included in system.h and in source files using it.. sometimes

i think it should be consistently before system.h (i'm not sure what's
going on with the INCLUDE_STRING macro), fortunately not many files are
affected in gcc/:


i can prepare a patch moving the c++ includes up and i'm open to
other suggestions. (including libc headers is also problematic because
of _GNU_SOURCE, but still safer than what is happening in c++ land
where #include <map> makes all locale.h, pthread.h, time.h, sched.h,
etc symbols visible).

x86_64-linux-musl-g++ -fno-PIE -c   -g -O2 -DIN_GCC     -fno-exceptions -fno-rtti -fasynchronous-unwind-tables
-W -Wall -Wno-narrowing -Wwrite-strings -Wcast-qual -Wmissing-format-attribute -Woverloaded-virtual -pedantic
-Wno-long-long -Wno-variadic-macros -Wno-overlength-strings   -DHAVE_CONFIG_H -I. -I. -I/src/gcc/gcc
-I/src/gcc/gcc/. -I/src/gcc/gcc/../include -I/src/gcc/gcc/../libcpp/include -I/build/host-tools/include
-I/build/host-tools/include -I/build/host-tools/include  -I/src/gcc/gcc/../libdecnumber
-I/src/gcc/gcc/../libdecnumber/dpd -I../libdecnumber -I/src/gcc/gcc/../libbacktrace   -o auto-profile.o -MT
auto-profile.o -MMD -MP -MF ./.deps/auto-profile.TPo /src/gcc/gcc/auto-profile.c
In file included from /tool/x86_64-linux-musl/include/pthread.h:30:0,
                 from /tool/x86_64-linux-musl/include/c++/6.0.1/x86_64-linux-musl/bits/gthr-default.h:35,
                 from /tool/x86_64-linux-musl/include/c++/6.0.1/x86_64-linux-musl/bits/gthr.h:148,
                 from /tool/x86_64-linux-musl/include/c++/6.0.1/ext/atomicity.h:35,
                 from /tool/x86_64-linux-musl/include/c++/6.0.1/bits/basic_string.h:39,
                 from /tool/x86_64-linux-musl/include/c++/6.0.1/string:52,
                 from /tool/x86_64-linux-musl/include/c++/6.0.1/stdexcept:39,
                 from /tool/x86_64-linux-musl/include/c++/6.0.1/array:39,
                 from /tool/x86_64-linux-musl/include/c++/6.0.1/tuple:39,
                 from /tool/x86_64-linux-musl/include/c++/6.0.1/bits/stl_map.h:63,
                 from /tool/x86_64-linux-musl/include/c++/6.0.1/map:61,
                 from /src/gcc/gcc/auto-profile.c:36:
/tool/x86_64-linux-musl/include/sched.h:74:7: error: attempt to use poisoned "calloc"
 void *calloc(size_t, size_t);
/tool/x86_64-linux-musl/include/sched.h:114:36: error: attempt to use poisoned "calloc"
 #define CPU_ALLOC(n) ((cpu_set_t *)calloc(1,CPU_ALLOC_SIZE(n)))

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