[ruby-core:102559] [Ruby master Misc#17637] Endless ranges with `nil` boundary weird behavior

Issue #17637 has been updated by zverok (Victor Shepelev).


I believe that using `nil` as a signifier of the "open end" is a compromise due to Ruby's ranges polymorphism. You can have range from a string, from time, from date, from any custom comparable class, how you'd signify the "open end" in this case? In some statically typed language it probably could've been some `Infinity<Type>`, but in Ruby... IDK, maybe the alternative would be some "generic" `Infinity` constant/special value (as incompatible with any type as `nil` is, but having different semantics), but it would be too large a change.

----------------------------------------
Misc #17637: Endless ranges with `nil` boundary weird behavior
https://bugs.ruby-lang.org/issues/17637#change-90462

* Author: gud (gud gud)
* Status: Open
* Priority: Normal
----------------------------------------

Basically it's about this https://andycroll.com/ruby/watch-out-for-nils-in-ranges/

Since Ruby 2.6 we have this weird syntax (0..nil) which is really really bug prone

e.g. we have dynamic upper boundary like

```
lower = 0
upper = some_method(arg1, arg2)

(lower..upper).each do { |s| some_method2(s) }
```

We rarely do `nil` checks in Ruby so it's really easy to have Infinity loop in the end.
Previous Argument error was more intuitive since it throws exception instead of silently looping forever.

+ some additional strange behavior:

```
(0..nil).count
=> Infinity

(0..Float::INFINITY).count
=> hangs, I guess same infinity loop
```

Having explicit parameter `Float::INFINITY` (as in previous versions) looks more like a proper design instead of allowing `nil` as a valid parameter.

You may think of it as **I would like to have a range from 0 to nothing, what is it actually ?**
And I guess the answer is **Nothing**.
Fixing `(0..Float::INFINITY).count` this case it also important I believe.

Tested on `ruby 2.7.1p83`



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