CPPFLAGS="-D_FORTIFY_SOURCE=2" breaks compiler-rt with clang++, only with gnu c++: realpath redefinition

[Quuxplusone]

Bugzilla Link	PR47163
Status	NEW
Importance	P release blocker
Reported by	solo turn (soloturn@gmail.com)
Reported on	2020-08-13 19:54:18 -0700
Last modified on	2020-09-12 12:42:42 -0700
Version	unspecified
Hardware	PC Linux
CC	llvm-bugs@lists.llvm.org
Fixed by commit(s)
Attachments
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Blocked by
See also

am trying to create a swift-lang package for arch linux, with a continuous
build here: https://github.com/soloturn/swift-aur/actions

funny is that on arch linux which first has newest packages, and second, dev
packages do not exist, there is just one package including the developer
version in many cases.

compiler-rt can be built with g++, but using clang++ fails

$ export CC=clang
$ export CC=clang
$ makepkg -s

and it really gives the same error when compiling swift.

/usr/bin/clang++  -I../lib/tsan/dd/../.. -D_FORTIFY_SOURCE=2 -march=x86-64 -
mtune=generic -O2 -pipe -fno-plt -Wall -std=c++14 -Wno-unused-parameter -O3 -
DNDEBUG  -m64 -fno-lto -fPIC -fno-builtin -fno-exceptions -fomit-frame-pointer -
funwind-tables -fno-stack-protector -fno-sanitize=safe-stack -
fvisibility=hidden -fno-lto -O3 -gline-tables-only -Wno-gnu -Wno-variadic-
macros -Wno-c99-extensions -Wno-non-virtual-dtor -fno-rtti -MD -MT
lib/tsan/dd/CMakeFiles/clang_rt.dd-x86_64.dir/dd_interceptors.cpp.o -MF
lib/tsan/dd/CMakeFiles/clang_rt.dd-x86_64.dir/dd_interceptors.cpp.o.d -o
lib/tsan/dd/CMakeFiles/clang_rt.dd-x86_64.dir/dd_interceptors.cpp.o -c
../lib/tsan/dd/dd_interceptors.cpp
../lib/tsan/dd/dd_interceptors.cpp:225:1: warning: attribute declaration must
precede definition [-Wignored-attributes]

__NTH (realpath (const char *__restrict __name, char *__restrict __resolved))
       ^
../lib/tsan/dd/dd_interceptors.cpp:225:20: error: redefinition of 'realpath'

see here the original bug report towards swift:
https://bugs.swift.org/browse/SR-13082

[Quuxplusone]

the export needs to be

$ export CC=clang
$ export CXX=clang++
$ makepkg -s

then it fails. if the variables are not set it takes the default, g++, and
compiles.

the command executed can be seen in the PKGBUILD file,
https://github.com/archlinux/svntogit-packages/blob/packages/compiler-rt/trunk/PKGBUILD

cd "$srcdir/$pkgname-$pkgver.src/build"
cmake .. -G Ninja \
  -DCMAKE_BUILD_TYPE=Release \
  -DCMAKE_INSTALL_PREFIX=/usr
ninja

[Quuxplusone]

unsetting the arch linux default flag:
CPPFLAGS="-D_FORTIFY_SOURCE=2"

makes it build.