Versions checked: 4.10.0, 7.2.1, Master (f1685926ca) (reproduces on all)

Repro steps (HDRP or Universal):
1. Create a PBR graph
2. Create a Position node
3. Create a DDX node
4. Plug the output of Position into DDX
5. Plug the output of DDX into Vertex Position

Expected result: you are blocked from plugging the output of DDX into Vertex Position, like you would for Sample Texture 2D
Actual result: you are allowed to plug it in, resulting in a shader compile error