`non_empty_list/2` tail absorption unexpected behavior

[lukaszsamson]

Existing issue

• I have searched existing issues and could not find a duplicate.

Elixir and Erlang/OTP versions

Erlang/OTP 28 [erts-16.4.0.1] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [jit]

Interactive Elixir (1.21.0-dev)

Operating system

any

Current behavior

I understand that type widening and tail absorption is deliberate but it leads to surprising results. Here I construct a type [none(), (...), | term()] which is not inhabited by any value. none() gets overapproximated to term()

Repro:

t = non_empty_list(none(), term())
empty?(t) # false, I'd expect true
to_quoted_string(t) # "non_empty_list(term(), term())"
equal?(non_empty_list(none(), term()), non_empty_list(term(), term())) # true

Expected behavior

I don't know. Either accepted overapproximation or improved precision

[gldubc]

I agree it's not ideal. I'm working on a new constructor for lists which won't have this issue.