Operating system: windows 10
Graphics card: gtx 970
Exact steps for others to reproduce the error
Default cube with Glass BSDF Shader and set roughness to zero (Use Principled BSDF to make glass give the same result).
Use a sun light with 0 degree angle to illuminate the scene.
The glass material in render result still has some roughness. The object in the glass is not lit enough and the shadow is too soft.