ambiguous overload for ‘operator<<' Error.

[kafi350]

I am facing this error after including vexcl. Even if i specify the backend it shows the error.
#include <vexcl/vexcl.hpp>

    ```

from /home/kafi/Documents/Projects/CODD-pro-lib_integration/main.cpp:14:
/usr/include/boost/date_time/date_generators.hpp: In member function ‘virtual std::string boost::date_time::nth_kday_of_month<date_type>::to_string() const’:
/usr/include/boost/date_time/date_generators.hpp:237:8: error: ambiguous overload for ‘operator<<’ (operand types are ‘std::basic_ostream’ and ‘int’)
236 | ss << 'M'
| ~~~~~~~~~
| |
| std::basic_ostream
237 | << static_cast(month_) << '.'
| ^~ ~~~~~~~~~~~~~~~~~~~~~~~~
| |
| int
In file included from /usr/include/c++/9/iterator:64,
from /usr/include/CL/cl.hpp:219,

[ddemidov]

I can not reproduce this with g++ 11. The following compiles for me:

#include <vexcl/vexcl.hpp>

int main() {
    std::cout << 'M' << std::endl;
}