I hate to ask this now that the code is all written, but is there a compelling reason why this couldn't have been implemented in ATen? (I guess, no easy way to insert the checks into the generated code?)

Good point, I hadn't thought about implementing the shape checks in aten. I think it shouldn't be too bad to do that: I could rename cat in the cwrap to something and implement a native function that wraps the renamed cat along with performing size checks.

The two downsides I see with this approach are that:

• tensors don't benefit from the size check (only Variables, since size check is in aten). When tensors/Variables get merged this won't be a problem
• we'd be doing an extra pass through the list of tensors to cat. Right now the size of the final tensor is computed at the same time as the size checks; separating the two would mean two loops, one for the size checks (in aten), and another to compute the size of the final tensor (in TH). This probably doesn't matter.

Alternatively I could just rewrite cat completely as a native function

we should keep them in TH/THC. there's no upside (now that the code is written) to do this in ATen.