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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 16:55:00 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100017

--- Comment #57 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
I don't see how the patch could be correct.
Looking at RAW_CXX_FOR_TARGET in configure, I see:
(starting with the line you're proposing to change):
RAW_CXX_FOR_TARGET=3D"$CXX_FOR_TARGET"
...
{ $as_echo "$as_me:${as_lineno-$LINENO}: checking where to find the target =
c++
for libstdc++" >&5
$as_echo_n "checking where to find the target c++ for libstdc++... " >&6; }
if test "x${build}" !=3D "x${host}" ; then
  if expr "x$RAW_CXX_FOR_TARGET" : "x/" > /dev/null; then
    # We already found the complete path
    ac_dir=3D`dirname $RAW_CXX_FOR_TARGET`
    { $as_echo "$as_me:${as_lineno-$LINENO}: result: pre-installed in $ac_d=
ir"
>&5
$as_echo "pre-installed in $ac_dir" >&6; }
  else
    # Canadian cross, just use what we found
    { $as_echo "$as_me:${as_lineno-$LINENO}: result: pre-installed" >&5
$as_echo "pre-installed" >&6; }
  fi
else

So, if CXX_FOR_TARGET starts with /, e.g. /opt/gcc-11.1/bin/g++ , then
the above will use dirname /opt/gcc-11.1/bin/g++ and set ac_dir to
/opt/gcc-11.1/bin
But with the proposed change, $RAW_CXX_FOR_TARGET will be /opt/gcc-11.1/bin=
/g++
-nostdinc++ and dirname will fail miserably and not set ac_dir to anything:
dirname /opt/gcc-11.1/bin/g++ -nostdinc++
dirname: invalid option -- 'n'
Try 'dirname --help' for more information.=