From: "mame (Yusuke Endoh) via ruby-core" <ruby-core@...>
Date: 2025-10-29T01:52:56+00:00
Subject: [ruby-core:123580] [Ruby Bug#21513] Converting endless range to set hangs

Issue #21513 has been updated by mame (Yusuke Endoh).

Status changed from Closed to Open
Assignee set to knu (Akinori MUSHA)

The fix introduced another inconsistency:

```
$ ruby -e 'Enumerator.new(Float::INFINITY) {|g| loop { g << 1 } }.to_set'
-e:1:in 'Enumerable#to_set': cannot initialize Set from an object with infinite size (ArgumentError)
        from -e:1:in '<main>'

$ ruby -e 'Enumerator.new(Float::INFINITY) {|g| loop { g << 1 } }.to_a # hungs'
```

I think checking if `Range#end` is nil is good enough, instead of calling `#size`. Note that calling `#size` caused another practical issue #21654.

@knu What do you think?

----------------------------------------
Bug #21513: Converting endless range to set hangs
https://bugs.ruby-lang.org/issues/21513#change-114962

* Author: viralpraxis (Iaroslav Kurbatov)
* Status: Open
* Assignee: knu (Akinori MUSHA)
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
Converting endless range to array raises:

``` shell
ruby -e '(1..).to_a'
-e:1:in 'Range#to_a': cannot convert endless range to an array (RangeError)
	from -e:1:in '<main>'
```

but converting to set does not:

``` shell
ruby -e '(1..).to_set # hangs'
```

I think it makes to raise in both cases for consistency. Something like this should do the trick:

``` diff
diff --git i/prelude.rb w/prelude.rb
index f49cada637..11bfa3fc95 100644
--- i/prelude.rb
+++ w/prelude.rb
@@ -30,6 +30,10 @@ module Enumerable
   # Makes a set from the enumerable object with given arguments.
   # Passing arguments to this method is deprecated.
   def to_set(*args, &block)
+    if self.class == Range && self.end.nil?
+      raise RangeError, "cannot convert endless range to a set"
+    end
+
     klass = if args.empty?
       Set
     else
diff --git i/test/ruby/test_range.rb w/test/ruby/test_range.rb
index f875c0ab40..27b968641b 100644
--- i/test/ruby/test_range.rb
+++ w/test/ruby/test_range.rb
@@ -1541,4 +1541,11 @@ def test_overlap?
     assert_not_operator((1...3), :overlap?, (3..4))
     assert_not_operator((...3), :overlap?, (3..))
   end
+
+  def test_to_set
+    assert_equal(Set[], (1..-1).to_set)
+    assert_equal(Set[1, 2, 3], (1..3).to_set)
+
+    assert_raise(RangeError) { (239..).to_set }
+  end
 end
```

If this patch is accepted, I'll open a PR.

``` shell
./ruby -v
ruby 3.5.0dev (2025-07-14T20:34:32Z master a6d483971a) +PRISM [x86_64-linux]
```




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