I have corrected the code as below, please merge.

void wstring_to_utf8(fmt::wmemory_buffer const& w_mem_buffer, fmt::memory_buffer& mem_buffer)
{
auto remain_space = static_cast<int32_t>(mem_buffer.capacity() - mem_buffer.size());
auto bytes_needed = remain_space;

// TODO: so far utf-8 will use at most 4 bytes for a character.
if ((w_mem_buffer.size() + 1) * 4 > static_cast<size_t>(remain_space))
{
// if our given string is larger than the capacity, calculate how many bytes we need
bytes_needed = ::WideCharToMultiByte(
CP_UTF8, 0, w_mem_buffer.data(), static_cast(w_mem_buffer.size()), NULL, 0, NULL, NULL);

mem_buffer.reserve(static_cast<uint32_t>(bytes_needed + 1 + mem_buffer.size())); // +1 for EOL symbol, i.e. '\n'

}

if (QUILL_UNLIKELY(bytes_needed == 0))
{
auto const error = std::error_code(GetLastError(), std::system_category());
std::ostringstream error_msg;
error_msg << "wstring_to_utf8 failed with error message "
<< """ << error.message() << "", errno "" << error.value() << """;
QUILL_THROW(QuillError{error_msg.str()});
}

// convert
bytes_needed =
::WideCharToMultiByte(CP_UTF8, 0, w_mem_buffer.data(), static_cast(w_mem_buffer.size()),
mem_buffer.data() + mem_buffer.size(), bytes_needed, NULL, NULL);

if (QUILL_UNLIKELY(bytes_needed == 0))
{
auto const error = std::error_code(GetLastError(), std::system_category());
std::ostringstream error_msg;
error_msg << "wstring_to_utf8 failed with error message "
<< """ << error.message() << "", errno "" << error.value() << """;
QUILL_THROW(QuillError{error_msg.str()});
}

// set the new size
mem_buffer.resize(static_cast<uint32_t>(bytes_needed + mem_buffer.size()));
}