My view on semantics is that since we allow things like 0-bit signals (at quite some effort), we should allow 0-length arrays, too. The shape of a proxy into a 0-length array is always (0,False), and during legalization, it is treated like Cat() on both LHS and RHS.

Note that we cannot simply elide all code that refers to a 0-length array during legalization. For example, one could do something like Cat(empty_array[index_signal], lhs_signal).eq(1) and this would still need to assign lhs_signal.

An alternative would be to forbid creating a proxy into a 0-length array.

Turns out that's the semantics everything already implements, with the exception of a bug in RTLIL backend.