Created attachment 31112 [details]
Source file with complete test case

g++ is not able to compile the following test case which you also can find in the attachment (file: 'cast.cc').

struct A
{
	virtual void do_it() const = 0;
};

struct B :  A
{
	virtual void do_it() const {}
};

struct C
{
	operator B() const { return B(); }
};

void do_it(const A& a) { a.do_it(); }

int main()
{
	C c;
	do_it(c);
	return 0;
}


When compiling this test case with clang++ (version 3.3) or the intel c++ compiler (versions 12.1 and 13.1), compilation succeeds without errors. When compiling the same test case with g++ (version 4.8.1 from MacPorts, running OSX Mavericks) the following errors are shown:

cast.cc: In function 'int main()':
cast.cc:21:9: error: cannot allocate an object of abstract type 'A'
 do_it(c);
        ^
cast.cc:1:8: note:   because the following virtual functions are pure within 'A':
struct A
       ^
cast.cc:3:15: note: 	virtual void A::do_it() const
 virtual void do_it() const = 0;

              ^

I get the same errors when i try to compile the test case with g++ (versions 4.8.1, 4.7.0, 4.6.1, 4.3.4, 3.3.6) running on SUSE Linux.

Basically I have an abstract base class A and some implementation B. Another class C provides a conversion operator to B. When passing instances of C to functions that take const references of A clang++ and icpc is able to find the conversion to B and is then able to convert the temporary instance of B to a const reference of A. I think, in the same case, g++ tries to make a temporary instance of A initialized with the instance of C which is not possible. 

A workaround for this problem is to explicitly convert the instance of C to B like this:

do_it((B)c);

But then it would be nice if this is not necessary.

Here is the output of gcc-mp-4.8 -v
Using built-in specs.
COLLECT_GCC=gcc-mp-4.8
COLLECT_LTO_WRAPPER=/opt/local/libexec/gcc/x86_64-apple-darwin13/4.8.1/lto-wrapper
Target: x86_64-apple-darwin13
Configured with: /opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_lang_gcc48/gcc48/work/gcc-4.8.1/configure --prefix=/opt/local --build=x86_64-apple-darwin13 --enable-languages=c,c++,objc,obj-c++,lto,fortran,java --libdir=/opt/local/lib/gcc48 --includedir=/opt/local/include/gcc48 --infodir=/opt/local/share/info --mandir=/opt/local/share/man --datarootdir=/opt/local/share/gcc-4.8 --with-local-prefix=/opt/local --with-system-zlib --disable-nls --program-suffix=-mp-4.8 --with-gxx-include-dir=/opt/local/include/gcc48/c++/ --with-gmp=/opt/local --with-mpfr=/opt/local --with-mpc=/opt/local --with-cloog=/opt/local --enable-cloog-backend=isl --disable-cloog-version-check --enable-stage1-checking --disable-multilib --enable-lto --enable-libstdcxx-time --with-as=/opt/local/bin/as --with-ld=/opt/local/bin/ld --with-ar=/opt/local/bin/ar --with-bugurl=https://trac.macports.org/newticket --with-pkgversion='MacPorts gcc48 4.8.1_3'
Thread model: posix
gcc version 4.8.1 (MacPorts gcc48 4.8.1_3)