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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/100017] [11/12 regression] error: 'fenv_t' has not
 been declared in '::' -- canadian compilation fails
Date: Thu, 06 Jan 2022 21:00:18 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100017

--- Comment #72 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Guess another option would be to do that
RAW_CXX_FLAGS_FOR_TARGET=3D"$RAW_CXX_FLAGS_FOR_TARGET -nostdinc++"
in the toplevel configure.ac after all the GCC_TARGET_TOOL lines.
While GCC_TARGET_TOOL does AC_SUBST on the second argument, so
AC_SUBST(RAW_CXX_FLAGS_FOR_TARGET), in autoconf AC_SUBST just remembers
that it should be substituted and does the substitutions late, so when the =
var
is changed in between it will propagate to the Makefiles etc.

--- configure.ac        2022-01-04 23:11:22.153595595 +0100
+++ configure.ac        2022-01-06 21:59:49.274985505 +0100
@@ -3669,6 +3669,8 @@ GCC_TARGET_TOOL(strip, STRIP_FOR_TARGET,
 GCC_TARGET_TOOL(windres, WINDRES_FOR_TARGET, WINDRES, [binutils/windres])
 GCC_TARGET_TOOL(windmc, WINDMC_FOR_TARGET, WINDMC, [binutils/windmc])

+RAW_CXX_FOR_TARGET=3D"$RAW_CXX_FOR_TARGET -nostdinc++"
+
 AC_SUBST(FLAGS_FOR_TARGET)
 AC_SUBST(RAW_CXX_FOR_TARGET)


completely untested...=