BaseDevice::init looks up the address for vkGetPhysicalDevicePresentRectanglesKHR:
m_khr_device_group_extension_entrypoints.vkGetPhysicalDevicePresentRectanglesKHR = reinterpret_cast<PFN_vkGetPhysicalDevicePresentRectanglesKHR>(get_proc_address("vkGetPhysicalDevicePresentRectanglesKHR") );
I haven't been able to test it myself, but I've got a report indicating that NULL is being returned here on a Radeon R9 Fury (latest drivers) which causes this assertion to fail:
anvil_assert(m_khr_device_group_extension_entrypoints.vkGetPhysicalDevicePresentRectanglesKHR != nullptr);
I've disabled the assertion check and everything seems to run fine, but I'm not sure what this function is for exactly, so I haven't created a pull request.