[ruby-core:108213] [Ruby master Bug#18155] (nil..nil).cover?(x) is true for all x since beginless ranges were introduced

Issue #18155 has been updated by sawa (Tsuyoshi Sawada).


I do not understand the logic behind option 2. I would rather expect this logic:

> a beginless-and-endless range has no type, so it imposes no restriction on the type

which would lead us to option 1. Its coverage should return `true` for any object.

To my understanding, the begin value restricts the range by imposing a minimum (type and value), and the end value restricts the range by imposing a maximum (type and value). Lacking one of them means it is restricted only in one end, lacking both of them should mean it is restricted in neither end.


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Bug #18155: (nil..nil).cover?(x) is true for all x since beginless ranges were introduced
https://bugs.ruby-lang.org/issues/18155#change-97196

* Author: urbanautomaton (Simon Coffey)
* Status: Open
* Priority: Normal
* ruby -v: 2.7.0 and greater
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
### Description

The introduction of beginless ranges in #14799 changed the behaviour of `Range#cover?` when both the beginning and the end of a `Range` are `nil`. Previously such a range would cover no values; now it covers all values:

```
irb-2.6.6> (nil..nil).cover?("23 dogs")
=> false

irb-2.7.0> (nil..nil).cover?("23 dogs")
=> true
```

This change was [noted at the time](https://bugs.ruby-lang.org/issues/14799#note-12), but it was believed that the call would have previously raised an error instead of returning `false`. I've not exhaustively checked, but the old behaviour goes back to at least ruby 2.2.

### Discussion

I'm not sure that either of these results are "correct", since I'm not sure that the concept of a range with no beginning or end is meaningful. Without either parameter, it seems impossible to infer the values it ranges over, since we have no type information available.

However, I did find both the current result and the change in behaviour quite surprising, so I've opened this issue.

Some options I considered:

1. **Do nothing:** a beginless-and-endless range is unbounded in both value and type, so covers all values of all types
2. **Reinstate the old behaviour:** a beginless-and-endless range has no type, so covers no values
3. **Raise an error:** a beginless-and-endless range is meaningless, so constructing one should raise an error

My personal preference is option 3, but I can clearly see that it represents the most effort. I'd welcome people's thoughts; if option 1 is preferred, I'd be happy to contribute a documentation patch to make the new behaviour clear.

---Files--------------------------------
range-18155.patch (1.08 KB)


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