Well... it works if I split uniform float2 worldDimensions; into the more verbose uniform float worldX; uniform float worldY; Which is very annoying. Looking inside the <simd/simd.h> it looks empty? Like all these structs in <simd/vector_types.h> have been taken out??

Okay it looks like things have changed a bit, and a little reading of the original instructions at: https://developer.apple.com/documentation/scenekit/scnshadable#1654834 have cleared things up for me. In short, my float2 should be described as: #pragma arguments float2 worldDimensions; and after that the #pragma body #pragma transparent The shader now works fine again. Also, the SIMD files are presumably empty because they are deprecated in favor of just using the float2, float4 Metal types directly.