[ruby-core:101331] [Ruby master Bug#17218] Range#step sometimes behaves unexpectedly with Rational endpoints and increment

Issue #17218 has been updated by mrkn (Kenta Murata).


@jeremyevans0 Thank you for making a candidate patch.  I have reviewed it, I noticed that it resolves only an issue of `last` method, but does not of `to_a` method.
I made another patch to resolve both issues: https://github.com/ruby/ruby/pull/3870

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Bug #17218: Range#step sometimes behaves unexpectedly with Rational endpoints and increment
https://bugs.ruby-lang.org/issues/17218#change-89029

* Author: rorymolinari (Rory Molinari)
* Status: Assigned
* Priority: Normal
* Assignee: mrkn (Kenta Murata)
* ruby -v: ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-darwin19]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
I am using the latest stable version. The same behavior exists in 2.6.6.
```
11:28:25 $ ruby -v                                                                                                                                                                                                                    
ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-darwin19]
```

When creating an `Enumerator::ArithmeticSequence` with Rational endpoints and increment, sometimes inconsistent behavior results due to floating-point approximation in `#last`.

``` ruby
x = Rational(10997, 10000)
y = Rational(11, 10)
s = Rational(1, 10000)

puts "#{[x, y, s].map(&:to_f)}" # -> [1.0997, 1.1, 0.0001]

# intention: this contains exactly the precise Rational representations of 1.0997, 1.0998, 1.0999, 1.1
arith_seq = (x..y).step(s)

puts arith_seq.class # -> Enumerator::ArithmeticSequence

# Things look OK
puts arith_seq.first # -> 10997/10000
puts arith_seq.end  # -> 11/10
puts arith_seq.step  # -> 1/10000

# But the array that we get from #to_a is missing the last element, (11/10)
puts arith_seq.to_a.to_s # -> [(10997/10000), (5499/5000), (10999/10000)]

# This is apparently due to the value of #last
puts arith_seq.last # -> 1.0999999999999999

# The object itself is confused
puts arith_seq.size # -> 4
puts arith_seq.to_a.size # -> 3
```

The issue is in the `Enumerator::ArithmeticSequence` instance we get when we call `Range#step` without a block. The `Range#step` method passes the right things when it gets a block.
``` ruby
block_vals = []
(x..y).step(s) { |v| block_vals << v }
puts block_vals.to_s # -> [(10997/10000), (5499/5000), (10999/10000), (11/10)]
```

I would expect `arith_seq.last` to be the exact value `Rational(11, 10)`. After all, `arith_seq` was created from a `Range` with `Rational` endpoints and given a rational step size.



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