Add infinity pattern

[ksss]

Add a pattern to Array#cycle that returns an Enumerator that loops infinitely, as it is missing.

Whether it loops infinitely or not depends on the receiver and cannot be isolated by overloading.

[1,2,3].cycle.each {} # Return type should be `bot` since it will never return

[].cycle.each {} # Just return `nil`

[soutaro]

@ksss I'm not sure if adding the bot case here. We don't have bot type with IO#gets while it doesn't return when the stream is closed. (I'm curious if having it improves something, and open to have it if it does.)

[ksss]

The key point of this issue is that it's not about bot itself, but about the type argument. There is no way to write a test case for [1,2,3].cycle despite it having a return value.

assert_send_type(
  "() -> Enumerator[untyped, untyped]",
  [1,2,3], :cycle
)
# Failure: test_cycle(ArrayInstanceTest):
#   Call trace does not match with given method type: #<struct RBS::Test::CallTrace method_name=:cycle, method_call=<RBS::Test::ArgumentsReturn:@arguments: [], @exit_value: #<Enumerator: [1, 2, 3]:cycle>, @exit_type: :return>, block_calls=[], block_given=false>.
#   <["[Array#cycle] ReturnTypeError: expected `Enumerator[untyped, untyped]` but returns `#<Enumerator: [1, 2, 3]:cycle>`"]> was expected to be empty.

---

Another perspective is that infinite loops can be a critical issue for applications. If programmers could determine the possibility of infinite loops from type information, they could avoid these problems. If it is known that there is a possibility of returning bot, programmers can avoid calling each from Enumerator[untyped, bot]. However, whether this applies to IO#gets is unclear.

[soutaro]

I still don't agree having the Enumerator[.., bot] case makes sense.

1. A method call may not return doesn't mean we should have bot return type.
2. We cannot use the two Enumerator union cases. We cannot decompose them with is_a?, and the each return type will be nil | bot, which is technically identical to nil.

[ksss]

OK make sense! Thank you for reviewing.