https://git.ruby-lang.org/ruby.git/commit/?id=5253b9579a

From 5253b9579a129f66a768dae24bd50a95bab02841 Mon Sep 17 00:00:00 2001
From: Victor Shepelev <zverok.offline@g...>
Date: Mon, 21 Dec 2020 02:32:30 +0200
Subject: Document usage of ArithmeticSequence in Array#slice, and add to NEWS
 (#3952)


diff --git a/NEWS.md b/NEWS.md
index d88642b..447788b 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -149,6 +149,14 @@ Outstanding ones only. https://github.com/ruby/ruby/blob/trunk/NEWS.md#L149
         * Array#uniq
         * Array#*
 
+    * Can be sliced with Enumerator::ArithmeticSequence
+
+        ```ruby
+        dirty_data = ['--', 'data1', '--', 'data2', '--', 'data3']
+        dirty_data[(1..).step(2)] # take each second element
+        # => ["data1", "data2", "data3"]
+        ```
+
 * ConditionVariable
 
     * ConditionVariable#wait may now invoke the `block`/`unblock` scheduler
diff --git a/array.c b/array.c
index f4c3e27..ea84473 100644
--- a/array.c
+++ b/array.c
@@ -1775,11 +1775,22 @@ static VALUE rb_ary_aref2(VALUE ary, VALUE b, VALUE e); https://github.com/ruby/ruby/blob/trunk/array.c#L1775
  *    a[4..0] # => nil
  *    a[4..-1] # => nil
  *
- *  When a single argument +aseq+ is given,
- *  ...(to be described)
- *
- *  Raises an exception if given a single argument
- *  that is not an \Integer-convertible object or a \Range object:
+ *  When a single Enumerator::ArithmeticSequence argument +aseq+ is given,
+ *  returns an Array of elements corresponding to the indexes produced by
+ *  the sequence.
+ *    a = ['--', 'data1', '--', 'data2', '--', 'data3']
+ *    a[(1..).step(2)] # => ["data1", "data2", "data3"]
+ *
+ *  Unlike slicing with range, if the start or the end of the arithmetic sequence
+ *  is larger than array size, throws RangeError.
+ *    a = ['--', 'data1', '--', 'data2', '--', 'data3']
+ *    a[(1..11).step(2)]
+ *    # RangeError (((1..11).step(2)) out of range)
+ *    a[(7..).step(2)]
+ *    # RangeError (((7..).step(2)) out of range)
+ *
+ *  If given a single argument, and its type is not one of the listed, tries to
+ *  convert it to Integer, and raises if it is impossible:
  *    a = [:foo, 'bar', 2]
  *    # Raises TypeError (no implicit conversion of Symbol into Integer):
  *    a[:foo]
diff --git a/enumerator.c b/enumerator.c
index 68cfc2b..fe5f054 100644
--- a/enumerator.c
+++ b/enumerator.c
@@ -3333,6 +3333,9 @@ enumerator_plus(VALUE obj, VALUE eobj) https://github.com/ruby/ruby/blob/trunk/enumerator.c#L3333
  * that is a representation of sequences of numbers with common difference.
  * Instances of this class can be generated by the Range#step and Numeric#step
  * methods.
+ *
+ * The class can be used for slicing Array (see Array#slice) or custom
+ * collections.
  */
 
 VALUE
-- 
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