Windows 7 SP1 (Fully Up-to-Date) with NVIDIA GTX 980 (Driver version: 387.92)
Broken: 2.79 5bd8ac9
Short description of error
When using the Principled BSDF and setting the Clearcoat to 1 and the Clearcoat Roughness to 0, the material turns black in the viewport (material shaded view).
Exact steps for others to reproduce the error
Add a Principled BSDF shader to any object, switch to the material shaded view in the viewport and set the Clearcoat to 1.0 and the Clearcoat Roughness to 0. It should turn black.
Please check the attached file.