After the embarrassment of driver 415.22 not actually compiling with Kernel 4.20, NVIDIA have now released 415.23.    Very brief details are here:  https://www.nvidia.com/Download/driverResults.aspx/141300/en-us

This version does compile with Kernel 4.20 (tested with 4.20-rc6) but shows repeated warnings:

.....................................
In file included from /home/rgadsdon/Downloads/NVIDIA-Linux-x86_64-415.23/kernel/nvidia-uvm/uvm8_gpu.h:31,
from /home/rgadsdon/Downloads/NVIDIA-Linux-x86_64-415.23/kernel/nvidia-uvm/uvm8_mmu_test.c:24:
/home/rgadsdon/Downloads/NVIDIA-Linux-x86_64-415.23/kernel/nvidia-uvm/uvm_linux.h:263:2: warning: #warning "atomic64_t unavailable, demoting to atomic_t!" [-Wcpp]
#warning "atomic64_t unavailable, demoting to atomic_t!"
^~~~~~~
CC [M] /home/rgadsdon/Downloads/NVIDIA-Linux-x86_64-415.23/kernel/nvidia-drm/nvidia-drm-helper.o
In file included from /home/rgadsdon/Downloads/NVIDIA-Linux-x86_64-415.23/kernel/nvidia-uvm/uvm8_test.h:27,
from /home/rgadsdon/Downloads/NVIDIA-Linux-x86_64-415.23/kernel/nvidia-uvm/uvm8_peer_identity_mappings_test.c:24:
/home/rgadsdon/Downloads/NVIDIA-Linux-x86_64-415.23/kernel/nvidia-uvm/uvm_linux.h:263:2: warning: #warning "atomic64_t unavailable, demoting to atomic_t!" [-Wcpp]
#warning "atomic64_t unavailable, demoting to atomic_t!"
^~~~~~~
CC [M] /home/rgadsdon/Downloads/NVIDIA-Linux-x86_64-415.23/kernel/nvidia-drm/nv-pci-table.o
CC [M] /home/rgadsdon/Downloads/NVIDIA-Linux-x86_64-415.23/kernel/nvidia-drm/nvidia-drm-gem-nvkms-memory.o
In file included from /home/rgadsdon/Downloads/NVIDIA-Linux-x86_64-415.23/kernel/nvidia-uvm/uvm_common.h:48,
from /home/rgadsdon/Downloads/NVIDIA-Linux-x86_64-415.23/kernel/nvidia-uvm/uvm8_va_block_test.c:24:
/home/rgadsdon/Downloads/NVIDIA-Linux-x86_64-415.23/kernel/nvidia-uvm/uvm_linux.h:263:2: warning: #warning "atomic64_t unavailable, demoting to atomic_t!" [-Wcpp]
#warning "atomic64_t unavailable, demoting to atomic_t!"
^~~~~~~
CC [M] /home/rgadsdon/Downloads/NVIDIA-Linux-x86_64-415.23/kernel/nvidia-drm/nvidia-drm-gem-user-memory.o
...................................

Despite this, the driver does load/run OK with Kernel 4.20-rc6 (Tested with KDE/Plasma etc..).

Robert Gadsdon.  December 13, 2018.