For example:

$ sail test.sail
Syntax error:
test.sail:1.10-10:
1 |type S = {-1}
  |          ^
  | current token: -

This was supported in 0.19. Sets containing only positive numbers are OK. The equivalent range(-1,-1) is fine.

This was triggered on an old version of the sail-riscv model that is still used by CHERIoT: CHERIoT-Platform/sail-riscv@f5e058c . The upstream sail-riscv repo has replaced this with the range syntax.