https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86860
Bug ID: 86860
Summary: Reject valid overloads of subclass ostream operator<<
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: mickey.veksler at gmail dot com
Target Milestone: ---
The following is rejected by gcc (not by other compilers):
#include <sstream>
#include <string>
struct St : std::ostringstream {
template<typename Tp> St& operator<<( const Tp& value ) { return *this; }
operator std::string() const { return str(); }
friend std::ostream& operator<<( std::ostream& s, const St& ss ) { return
s << ss.str(); }
};
struct Memory_type {
std::string to_string() const { return St() << "s=" << s; }
const char* s;
};
Because of an ambiguity between user defined:
template<typename Tp> St& operator<<( const Tp& value ) { return *this; }
and GCC function defined (in ostream header):
template<typename _Ostream, typename _Tp>
inline
typename enable_if<__and_<__not_<is_lvalue_reference<_Ostream>>,
__is_convertible_to_basic_ostream<_Ostream>,
__is_insertable<
__rvalue_ostream_type<_Ostream>,
const _Tp&>>::value,
__rvalue_ostream_type<_Ostream>>::type
operator<<(_Ostream&& __os, const _Tp& __x)
{ .... }
The above function does not seem to be part of the standard, and it seems that
the other compilers can work without it.
This is described in
https://stackoverflow.com/questions/51637953/what-enable-if-or-other-hint-is-need-for-the-following-overloaded-to-compile/51692662#51692662
