https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100017
--- Comment #77 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jonathan Wakely <r...@gcc.gnu.org>: https://gcc.gnu.org/g:4fde88e5dd152fe866a97b12e0f8229970d15cb3 commit r12-6406-g4fde88e5dd152fe866a97b12e0f8229970d15cb3 Author: Jonathan Wakely <jwak...@redhat.com> Date: Fri Jan 7 15:21:03 2022 +0000 libstdc++: Add -nostdinc++ for c++17 sources [PR100017] When building a build!=host compiler, the just-built gcc can't be used to build the target libstdc++ (because it is built for the host triplet, not the build triplet). The top-level configure.ac sets up the build flags for libstdc++ (and other "raw_cxx" libs) like this: GCC_TARGET_TOOL(c++ for libstdc++, RAW_CXX_FOR_TARGET, CXX, [gcc/xgcc -shared-libgcc -B$$r/$(HOST_SUBDIR)/gcc -nostdinc++ -L$$r/$(TARGET_SUBDIR)/libstdc++-v3/src -L$$r/$(TARGET_SUBDIR)/libstdc++-v3/src/.libs -L$$r/$(TARGET_SUBDIR)/libstdc++-v3/libsupc++/.libs], c++) The -nostdinc++ flag is only used for the IN-TREE-TOOL, i.e. when using the just-built gcc/xgcc compiler. This means that the cross-compiler used to build libstdc++ will add its own libstdc++ headers to the include path. That results in the #include <cfenv> in src/c++17/floating_to_chars.cc and src/c++17/floating_from_chars.cc doing #include_next <fenv.h> and finding the libstdc++ fenv.h wrapper from the host compiler. Because that has the same include guard as the <fenv.h> in the libstdc++ we're trying to build, we never reach the underlying <fenv.h> from libc. That results in several errors of the form: error: 'fenv_t' has not been declared in '::' The most correct fix would be to add -nostdinc++ to the RAW_CXX_FOR_TARGET variable in configure.ac, or the RAW_CXX_TARGET_EXPORTS variable in Makefile.tpl. Another solution would be to make the libstdc++ <fenv.h> wrapper use _GLIBCXX_INCLUDE_NEXT_C_HEADERS like our <stdlib.h> and other C header wrappers. For now though, the simplest and safest solution is to just add -nostdinc++ to the CXXFLAGS used for src/c++17/*.cc, which is what this does. libstdc++-v3/ChangeLog: PR libstdc++/100017 * src/c++17/Makefile.am (AM_CXXFLAGS): Add -nostdinc++. * src/c++17/Makefile.in: Regenerate.
