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#include <ctime>
#include <iostream>
#include <sstream>
#include <string>
 
struct make_string
{
    template <typename T>
    make_string & operator<<(const T & t)
    {
        ostr << t;
        return *this;
    }
 
    operator std::string() const { return ostr.str(); }
 
    std::string str() const { return ostr.str(); }
 
    std::ostringstream ostr;
};
 
 
void message(const std::string&) {}
 
 
int main()
{
    message(make_string() << "1 + 1 = " << 2 << "\n"); // OK
    message(make_string() << "1 + 1 = " << 2 << std::endl); // Not OK
 
 
    // OK
    std::ostringstream os;
    os << std::endl;
 
    // Not ok.
    make_string mk;
    mk << std::endl;
}

prog.cpp: In function ‘int main()’:
prog.cpp:29: error: no match for ‘operator<<’ in ‘((make_string*)((<anonymous> <unknown operator> {{{0u, {{0u, 0, 0, (std::_Ios_Fmtflags)0u, _S_goodbit, _S_goodbit, 0u, {0u, 0l}, {{0u, 0l}}, 0, 0u, {0u}}, 0u, '\000', false, 0u, 0u, 0u, 0u}}, {{0u, 0u, 0u, 0u, 0u, 0u, 0u, {0u}}, (std::_Ios_Openmode)0u, {{0u}}}, {{0u, 0, 0, (std::_Ios_Fmtflags)0u, _S_goodbit, _S_goodbit, 0u, {0u, 0l}, {{0u, 0l}}, 0, 0u, {0u}}, 0u, '\000', false, 0u, 0u, 0u, 0u}}}), make_string()).make_string::operator<< [with T = char [9]](((const char (&)[9])"1 + 1 = ")))->make_string::operator<< [with T = int](((const int&)((const int*)(&2)))) << std::endl’
prog.cpp:38: error: no match for ‘operator<<’ in ‘mk << std::endl’

Standard output is empty