Cycles renders black object with smooth shading and negative scale. Earlier was similar bug reports in Blender itself, but now this happens in Cycles.
I have managed to trace back and can offer possible fix:
in src/kernel/geom/geom_triangle.h, function triangle_smooth_normal you should invert normal vector if it is negative scale:
normalize((1.0f - u - v)*n2 + u*n0 + v*n1) => -normalize((1.0f - u - v)*n2 + u*n0 + v*n1)
I can't reproduce this issue, please provide all the information requested in the instructions, with .blend file, Blender version, etc.
We already have some code to handle cases like this, see usage of transform_negative_scale.
I always reproduce it but only on cycles alone (not on whole blender), here is project file
and I use commandbin/cycles project.xml
Should be fixed now, we didn't notice this was broken since we get the normals Blender from usually. The issue was that both the face normal and vertex normal were being flipped, which cancels each other out.
The fix is in the Blender repo, that gets synced into the Cycles repo at some point.