inline functions are required to be declared and defined in the same scope. The inline specifier changed the linkage signature an prevented this from compiling.
Undefined symbols for architecture x86_64: "vtkMRMLSliceNode::GetNumberOfThreeDViewIDs() const", referenced from: PyvtkMRMLSliceNode_GetNumberOfThreeDViewIDs(object, object) in vtkMRMLSliceNodePython.cxx.o ld: symbol(s) not found for architecture x86_64 clang: error: linker command failed with exit code 1 (use -v to see invocation)