C:\Temp>type example.cpp
#include <ios>

int main()
{
    auto open_flags = std::ios::binary;
    open_flags |= std::ios::app;

    return 0;
}

C:\Temp>cl -std:c++17 .\example.cpp

example.cpp
<source>(6): error C2678: binary '|=': no operator found which takes a left-hand operand of type 'std::_Iosb<int>::_Openmode' (or there is no acceptable conversion)
C:/data/msvc/14.21.27702.2/include\cstddef(70): note: could be 'std::byte &std::operator |=(std::byte &,const std::byte) noexcept' [found using argument-dependent lookup]
<source>(6): note: while trying to match the argument list '(std::_Iosb<int>::_Openmode, const std::_Iosb<int>::_Openmode)'
Compiler returned: 2