[ruby-core:99226] [Ruby master Bug#17017] Range#max & Range#minmax incorrectly use Float end as max

Issue #17017 has been updated by marcandre (Marc-Andre Lafortune).


Eregon (Benoit Daloze) wrote in #note-16:
> marcandre (Marc-Andre Lafortune) wrote in #note-15:
> > Unless I'm mistaken, this behavior change was not approved by Matz (or anybody else), changes a behavior that dates back to Ruby 1.8, breaks (that we know of) `activemodel` and `rubocop`, doesn't even raise the right error and isn't tested, ...
> 
> Which change is that?

I should have been more clear: by "this behavior change" I was referring exclusively to the change to `(1..Float::INFINITY).max`.

jeremyevans0 (Jeremy Evans) wrote in #note-13:
> ...obvious bug, which is that (1..2.1).max returned 2.1 instead of 2...
> Were you against fixing that bug?

Sorry I wasn't clear: I am not against that fix.


----------------------------------------
Bug #17017: Range#max & Range#minmax incorrectly use Float end as max
https://bugs.ruby-lang.org/issues/17017#change-86607

* Author: sambostock (Sam Bostock)
* Status: Open
* Priority: Normal
* ruby -v: ruby 2.8.0dev (2020-07-14T04:19:55Z master e60cd14d85) [x86_64-darwin17]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
While continuing to add edge cases to [`Range#minmax` specs](https://github.com/ruby/spec/pull/777), I discovered the following bug:

```ruby
(1..3.1).to_a        == [1, 2, 3] # As expected

(1..3.1).to_a.max    == 3         # As expected
(1..3.1).to_a.minmax == [1, 3]    # As expected

(1..3.1).max    == 3.1            # Should be 3, as above
(1..3.1).minmax == [1, 3.1]       # Should be [1, 3], as above
```

One way to detect this scenario might be to do (whatever the C equivalent is of)

```ruby
range_end.is_a?(Numeric)                      // Is this a numeric range?
  && (range_end - range_begin).modulo(1) == 0 // Can we reach the range_end using the standard step size (1)
```

As for how to handle it, a couple options come to mind:

- We could error out and do something similar to what we do for exclusive ranges

```ruby
raise TypeError, 'cannot exclude non Integer end value'
```

- We might be able to calculate the range end by doing something like

```ruby
num_steps = (range_end / range_beg).to_i - 1 # one fewer steps than would exceed the range_end
max = range_beg + num_steps                  # take that many steps all at once
```

- We could delegate to `super` and enumerate the range to find the max

```ruby
super
```

- We could update the documentation to define the max for this case as the `range_end`, similarly to how the documentation for `include?` says it behaves like `cover?` for numeric ranges.



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