I am referring specifically to geometric distortion, not wide angle perspective "stretching". The distortion I refer to is the failure of the lens to form a true rectilinear image due to asymmetry and the position of the diaphragm. These compromises cause either barrel distortion (straight lines "bow" outward, most visible along the edges of the frame), pincushion distortion (the opposite of barrel) or complex distortion (a combination of barrel and pincushion - typically barrel transitioning to pincushion toward the corners, hence the term "mustache" often used to describe it). Generally in retrofocus lenses barrel distortion is the common defect while pincushion distortion usually affects telephoto designs (to a lesser degree) although either type of distortion (or both) can be present in both long and short focal lengths.
Wide angle RF lenses typically suffer significantly less barrel distortion than SLR retrofocus wides.