[Bug c++/58923] New: g++ does not use provided conversion function for implicit conversion to another type

[andre.ritter at fau dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58923

            Bug ID: 58923
           Summary: g++ does not use provided conversion function for
                    implicit conversion to another type
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andre.ritter at fau dot de

Created attachment 31112
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=31112&action=edit
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)