[Bug libstdc++/99865] New: std::filesystem namespace identifiers pollute global namespace

["zoid at riseup dot net" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99865

            Bug ID: 99865
           Summary: std::filesystem namespace identifiers pollute global
                    namespace
           Product: gcc
           Version: 10.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zoid at riseup dot net
  Target Milestone: ---

At least two free functions from the std::filesystem namespace resolve without
being scope-resolved. The two I found were std::filesystem::create_directory
and std::filesystem::remove_all. There might be more. I am uncertain if this is
a problem in the library implementation or the compiler itself.

Minimal Reproducible Example:

#include <filesystem>

struct TempDir {
  std::filesystem::path const path;
  TempDir(std::filesystem::path const& p) : path(p) {
    // std::filesystem::create_directory(p); // <-- expected
    if (!create_directory(p)) { // <- compiles
      throw 0;
    }
  }
  ~TempDir() {
    // std::filesystem::remove_all(path); // <-- expected
    remove_all(path); // <- compiles
  }
};

int main() {
  // create_directory("bar"); // <- rejected (correctly)
  TempDir("foo");
}

Compile with: g++ -Wall -Wextra -pedantic -std=c++17 mre.cpp
Produces zero diagnostics. Compiles binary a.out.

My expectation would have been that the compiler rejected the lines marked with
"compiles". As it does when these identifiers appear directly in main.

Tested with g++ 10.2.1 under both debian and fedora. Output of g++ -v:

Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/10/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa:hsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 10.2.1-6'
--with-bugurl=file:///usr/share/doc/gcc-10/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-10
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib
--enable-libphobos-checking=release --with-target-system-zlib=auto
--enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686
--with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib
--with-tune=generic
--enable-offload-targets=nvptx-none=/build/gcc-10-Km9U7s/gcc-10-10.2.1/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-10-Km9U7s/gcc-10-10.2.1/debian/tmp-gcn/usr,hsa
--without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
--with-build-config=bootstrap-lto-lean --enable-link-mutex
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 10.2.1 20210110 (Debian 10.2.1-6)