Ubuntu 18.04, GTX 1080.
Broken: Blender 2.8 rB974d158
Short description of error
There is no way to bake multires displacement relative to a smoothly subdivided base mesh. This is what you want when applying displacement with Cycles and adaptive subdivision for example.
Exact steps for others to reproduce the error
Press "Bake" in the attached .blend, the mesh with displacement modifier should be identical to the multires mesh.