1 files changed, 37 insertions, 3 deletions
diff --git a/spec/ruby/core/array/each_spec.rb b/spec/ruby/core/array/each_spec.rb
index 256647d61e..57d6082f01 100644
--- a/spec/ruby/core/array/each_spec.rb
+++ b/spec/ruby/core/array/each_spec.rb
@@ -1,11 +1,13 @@
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 require_relative 'shared/enumeratorize'
+require_relative 'shared/iterable_and_tolerating_size_increasing'
 require_relative '../enumerable/shared/enumeratorized'
 
-# Modifying a collection while the contents are being iterated
-# gives undefined behavior. See
-# http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/23633
+# Mutating the array while it is being iterated is discouraged as it can result in confusing behavior.
+# Yet a Ruby implementation must not crash in such a case, and following the simple CRuby behavior makes sense.
+# CRuby simply reads the array storage and checks the size for every iteration;
+# like `i = 0; while i < size; yield self[i]; end`
 
 describe "Array#each" do
   it "yields each element to the block" do
@@ -15,6 +17,34 @@ describe "Array#each" do
     a.should == [1, 2, 3]
   end
 
+  it "yields each element to the block even if the array is changed during iteration" do
+    a = [1, 2, 3, 4, 5]
+    iterated = []
+    a.each { |x| iterated << x; a << x+5 if x.even? }
+    iterated.should == [1, 2, 3, 4, 5, 7, 9]
+  end
+
+  it "yields only elements that are still in the array" do
+    a = [0, 1, 2, 3, 4]
+    iterated = []
+    a.each { |x| iterated << x; a.pop if x.even? }
+    iterated.should == [0, 1, 2]
+  end
+
+  it "yields elements based on an internal index" do
+    a = [0, 1, 2, 3, 4]
+    iterated = []
+    a.each { |x| iterated << x; a.shift if x.even? }
+    iterated.should == [0, 2, 4]
+  end
+
+  it "yields the same element multiple times if inserting while iterating" do
+    a = [1, 2]
+    iterated = []
+    a.each { |x| iterated << x; a.unshift(0) if a.size == 2 }
+    iterated.should == [1, 1, 2]
+  end
+
   it "yields each element to a block that takes multiple arguments" do
     a = [[1, 2], :a, [3, 4]]
     b = []
@@ -46,3 +76,7 @@ describe "Array#each" do
   it_behaves_like :enumeratorize, :each
   it_behaves_like :enumeratorized_with_origin_size, :each, [1,2,3]
 end
+
+describe "Array#each" do
+  it_behaves_like :array_iterable_and_tolerating_size_increasing, :each
+end