https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80940
Bug ID: 80940
Summary: Private inheritance from std::ostream - compilation
error for custom operator <<
Product: gcc
Version: 7.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: witosx at gmail dot com
Target Milestone: ---
Even though custom operator <<(const char*) is defined compilation fails due to
the implementation of std::__is_convertible_to_basic_ostream trait. This breaks
code that compiled fine until libstdc++ 7.x
------------------------------------------------------------
------------------------------------------------------------
#include <ostream>
// Simple class uses private inheritance from std::ostream
class MyStream : private std::ostream
{
public:
MyStream& operator <<(const char*)
{
return *this;
}
};
int main()
{
MyStream stream{};
stream << "aaa"; // error in this line
}
------------------------------------------------------------
------------------------------------------------------------
/usr/include/c++/7.1.1/ostream: In instantiation of ‘struct
std::__is_convertible_to_basic_ostream<MyStream&>’:
/usr/include/c++/7.1.1/ostream:656:5: required by substitution of
‘template<class _Ostream, class _Tp> typename
std::enable_if<std::__and_<std::__not_<std::is_lvalue_reference<_Tp> >,
std::__is_convertible_to_basic_ostream<_Ostream>,
std::__is_insertable<_Ostream&, const _Tp&, void> >::value, typename
std::__is_convertible_to_basic_ostream<_Tp>::ostream_type>::type
std::operator<<(_Ostream&&, const _Tp&) [with _Ostream = MyStream&; _Tp = char
[4]]’
ls.cpp:40:15: required from here
/usr/include/c++/7.1.1/ostream:625:23: error: ‘std::basic_ostream<char>’ is an
inaccessible base of ‘MyStream’
decltype(__check(declval<typename remove_reference<_Tp>::type*>()));
~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/7.1.1/ostream:625:23: error: ‘std::basic_ostream<char>’ is an
inaccessible base of ‘MyStream’
------------------------------------------------------------
------------------------------------------------------------
The problem lies in
template<typename _Tp>
struct __is_convertible_to_basic_ostream
{
// !!! compiler picks this version of __check and fails !!!
template<typename _Ch, typename _Up>
static basic_ostream<_Ch, _Up>& __check(basic_ostream<_Ch, _Up>*);
static void __check(...);
...
};
Something like this works for me:
template<typename _Ch, typename _Up>
void __is_convertible_to_basic_ostream_test(basic_ostream<_Ch, _Up>*);
template<typename _Tp, typename = void_t<>>
struct __is_convertible_to_basic_ostream : false_type {};
template<typename _Tp>
struct __is_convertible_to_basic_ostream<_Tp,
void_t<decltype(__is_convertible_to_basic_ostream_test(declval<typename
remove_reference<_Tp>::type*>()))>> : true_type {};
------------------------------------------------------------
------------------------------------------------------------
$ gcc -v :(
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure --prefix=/usr --libdir=/usr/lib
--libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=https://bugs.archlinux.org/
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++ --enable-shared
--enable-threads=posix --enable-libmpx --with-system-zlib --with-isl
--enable-__cxa_atexit --disable-libunwind-exceptions --enable-clocale=gnu
--disable-libstdcxx-pch --disable-libssp --enable-gnu-unique-object
--enable-linker-build-id --enable-lto --enable-plugin
--enable-install-libiberty --with-linker-hash-style=gnu
--enable-gnu-indirect-function --disable-multilib --disable-werror
--enable-checking=release
Thread model: posix
gcc version 7.1.1 20170516 (GCC)
