[Bug c++/77747] New: GCC allows inheriting constructors from indirect base

["vasek.potocek at post dot cz" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77747

            Bug ID: 77747
           Summary: GCC allows inheriting constructors from indirect base
           Product: gcc
           Version: 6.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vasek.potocek at post dot cz
  Target Milestone: ---

Created attachment 39690
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=39690&action=edit
Minimal test case (preprocessed file)

According to the standard ([namespace.udecl] §3), it should only be possible to
inherit constructors from a direct base class. The following example shows a
good example why this is important:

class X {

public:
  X() { std::cout << "Default" << std::endl; }
  X(int a) { std::cout << a << std::endl; }
};

class Y : public X { };

class Z : public Y {
  using X::X;
};

int main() {
  Z instance{3};  // how is Y constructed?
}

G++ accepts this code even with strictest warning level settings with the
following result: Y is default-constructed, default-constructing X on its way,
and the parameter provided in the last line is silently ignored.

System version:

Target: x86_64-redhat-linux
Configured with: ../configure --enable-bootstrap
--enable-languages=c,c++,objc,obj-c++,fortran,ada,go,lto --prefix=/usr
--mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared
--enable-threads=posix --enable-checking=release --enable-multilib
--with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions
--enable-gnu-unique-object --enable-linker-build-id
--with-linker-hash-style=gnu --enable-plugin --enable-initfini-array
--disable-libgcj --with-isl --enable-libmpx --enable-gnu-indirect-function
--with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux
Thread model: posix
gcc version 6.2.1 20160916 (Red Hat 6.2.1-2) (GCC) 

Command line:

g++ -Weffc++ -Wall -Wextra -pedantic -fno-diagnostics-show-caret -save-temps
skip.cpp -o skip

Compiler output: none