We go into more details in the Limitations section of the paper, but in short Enzyme requires the following properties:

* IR of active functions must be accessible when Enzyme is called (e.g. cannot differentiate dlopen'd functions)

* Enzyme must be able to deduce the types of operations being performed (see paper section on interprocedural type analysis for details why)

* Support for exceptions is limited (and running with -fno-exceptions, equivalent in a diff language, or LLVM's exception lowering pass removes these).

* Support for parallel code (CPU/GPU) is ongoing [and see the prior comment on GPU parallelism for details]