Upstreaming: add -nostdinc++ to libstdc++ src/c++23 (Canadian-cross std module)

Upstreaming: add -nostdinc++ to libstdc++ src/c++23 (Canadian-cross std module)#

This documents the upstream form of the fix we carry downstream in linux/scripts/02-toolchain/build-gcc.sh. Our build patches the pre-generated Makefile.in at build time (no autotools needed in-container); upstream wants the change in the source Makefile.am plus a regenerated Makefile.in.

Related upstream bugs: PR libstdc++/100017, PR libstdc++/101060 (Canadian-cross #include_next <fenv.h> picks up the host libstdc++ wrapper, guard-collides on _GLIBCXX_FENV_H, so libc <fenv.h> is never reached).

Root cause (one paragraph)#

In a Canadian cross (build != host == target) the host g++’s libstdc++ headers are on the search path. libstdc++’s <fenv.h> wrapper (c_compatibility/fenv.h) guards itself with _GLIBCXX_FENV_H and does #include_next <fenv.h> to reach the libc header. When the target wrapper sets _GLIBCXX_FENV_H then #include_next lands on the host wrapper, that wrapper is guard-skipped, so its own #include_next never fires and the libc <fenv.h> is never included — ::fenv_t and every fe* become undeclared. The fix used everywhere else in libstdc++ is -nostdinc++, which keeps the host C++ headers off the search path. It is present in src/c++17/Makefile.am (that was the PR100017 fix) but was never propagated to src/c++23, which builds the C++23 std/std.compat modules from std.cc. Result: the module compile fails and libstdc++’s recipe silently ships an empty module (stamp-modules-bits ... Error 1 (ignored)). Also observed on native Apple Silicon (Homebrew homebrew-core#289142).

The patch (against GCC master / releases/gcc-16)#

--- a/libstdc++-v3/src/c++23/Makefile.am
+++ b/libstdc++-v3/src/c++23/Makefile.am
@@ AM_CXXFLAGS
 AM_CXXFLAGS = \
-	-std=gnu++23 \
+	-std=gnu++23 -nostdinc++ \
 	$(glibcxx_lt_pic_flag) $(glibcxx_compiler_shared_flag) \
 	$(XTEMPLATE_FLAGS) $(VTV_CXXFLAGS) \
 	$(WARN_CXXFLAGS) $(OPTIMIZE_CXXFLAGS) $(CONFIG_CXXFLAGS) \
 	-fimplicit-templates

Then regenerate src/c++23/Makefile.in (do not hand-edit it) with the exact autotools versions GCC pins — automake 1.15.1 / autoconf 2.69 — e.g. via contrib/config-list.mk/autoreconf in that tree, otherwise the diff will be full of spurious churn. The regenerated Makefile.in gets the same one-token change in its copied AM_CXXFLAGS.

This mirrors src/c++17/Makefile.am, which already reads -std=gnu++17 -nostdinc++, so it is a consistency fix, not a new idea.

Commit message / ChangeLog (GCC format)#

libstdc++: add -nostdinc++ when building the std module [PR100017]

The C++23 std/std.compat modules are compiled from src/c++23/std.cc with
AM_CXXFLAGS, but that variable lacked -nostdinc++, unlike src/c++17 (fixed in
PR100017).  In a Canadian cross the host libstdc++ <fenv.h> wrapper is then on
the include path; it shares the _GLIBCXX_FENV_H guard with the target wrapper,
so `#include_next <fenv.h>` never reaches the libc header, `::fenv_t` is
undeclared, std.cc fails to compile and an empty std module is shipped.

	PR libstdc++/100017
	PR libstdc++/101060

libstdc++-v3/ChangeLog:

	* src/c++23/Makefile.am (AM_CXXFLAGS): Add -nostdinc++, matching
	src/c++17.
	* src/c++23/Makefile.in: Regenerate.

Signed-off-by: Jonas Heinle <jonasheinle@googlemail.com>

Submission steps#

  1. Bugzilla: comment on PR100017/PR101060 (or file a new PR) noting src/c++23 never received the -nostdinc++ fix, with a minimal Canadian-cross repro (--build=x86_64-linux-gnu --host=aarch64-linux-gnu --target=aarch64-linux-gnu, --enable-languages=c,c++) and the fenv_t has not been declared output.

  2. Legal: GCC accepts the DCO (Signed-off-by:) for small contributions in lieu of FSF copyright assignment; a one-token flag change is well under the threshold. Keep the sign-off in the commit.

  3. Test: bootstrap + a Canadian-cross build showing std.cc compiles clean and the installed std.cc/std.gcm are non-empty; ideally make check-target-libstdc++-v3 for no regressions on a native build.

  4. Send: git format-patch -1 then git send-email to gcc-patches@gcc.gnu.org, CC the libstdc++ maintainers (see MAINTAINERS). Subject prefix [PATCH] libstdc++:.

Relationship to our downstream fix#

build-gcc.sh seds -nostdinc++ into the shipped Makefile.in before configure. Our guard ! grep -q -- '-nostdinc++' makes it a no-op the moment upstream lands this, so we can drop the downstream patch after bumping to a fixed GCC without any coordination. See linux/scripts/02-toolchain/build-gcc.sh and memory canadian-cross-fenv-module-noise.