change return value of `duplicable?` with Ruby 2.4+

`NilClass`, `FalseClass`, `TrueClass`, `Symbol` and `Numeric` can dup
with Ruby 2.4+.

Ref: https://bugs.ruby-lang.org/issues/12979

activesupport/CHANGELOG.md

+*   Change return value of `NilClass#duplicable?`, `FalseClass#duplicable?`,
+    `TrueClass#duplicable?`, `Symbol#duplicable?` and `Numeric#duplicable?`
+    to true with Ruby 2.4+. These classes can dup with Ruby 2.4+.
+
+    *Yuji Yaginuma*
+
 *   Remove deprecated class `ActiveSupport::Concurrency::Latch`
 
     *Andrew White*

activesupport/lib/active_support/core_ext/object/duplicable.rb

 #--
-# Most objects are cloneable, but not all. For example you can't dup +nil+:
+# Most objects are cloneable, but not all. For example you can't dup methods:
 #
-#   nil.dup # => TypeError: can't dup NilClass
+#   method(:puts).dup # => TypeError: allocator undefined for Method
 #
 # Classes may signal their instances are not duplicable removing +dup+/+clone+
 # or raising exceptions from them. So, to dup an arbitrary object you normally
@@ -19,7 +19,7 @@
 class Object
   # Can you safely dup this object?
   #
-  # False for +nil+, +false+, +true+, symbol, number, method objects;
+  # False for method objects;
   # true otherwise.
   def duplicable?
     true
@@ -27,6 +27,10 @@ def duplicable?
 end
 
 class NilClass
+  begin
+    nil.dup
+  rescue TypeError
+
     # +nil+ is not duplicable:
     #
     #   nil.duplicable? # => false
@@ -34,9 +38,14 @@ class NilClass
     def duplicable?
       false
     end
+  end
 end
 
 class FalseClass
+  begin
+    false.dup
+  rescue TypeError
+
     # +false+ is not duplicable:
     #
     #   false.duplicable? # => false
@@ -44,9 +53,14 @@ class FalseClass
     def duplicable?
       false
     end
+  end
 end
 
 class TrueClass
+  begin
+    true.dup
+  rescue TypeError
+
     # +true+ is not duplicable:
     #
     #   true.duplicable? # => false
@@ -54,9 +68,14 @@ class TrueClass
     def duplicable?
       false
     end
+  end
 end
 
 class Symbol
+  begin
+    :symbol.dup
+  rescue TypeError
+
     # Symbols are not duplicable:
     #
     #   :my_symbol.duplicable? # => false
@@ -64,9 +83,14 @@ class Symbol
     def duplicable?
       false
     end
+  end
 end
 
 class Numeric
+  begin
+    1.dup
+  rescue TypeError
+
     # Numbers are not duplicable:
     #
     #  3.duplicable? # => false
@@ -74,6 +98,7 @@ class Numeric
     def duplicable?
       false
     end
+  end
 end
 
 require "bigdecimal"

activesupport/test/core_ext/object/duplicable_test.rb

@@ -4,8 +4,13 @@
 require "active_support/core_ext/numeric/time"
 
 class DuplicableTest < ActiveSupport::TestCase
+  if RUBY_VERSION >= "2.4.0"
     RAISE_DUP = [method(:puts)]
+    ALLOW_DUP = ["1", Object.new, /foo/, [], {}, Time.now, Class.new, Module.new, BigDecimal.new("4.56"), nil, false, true, :symbol, 1, 2.3]
+  else
+    RAISE_DUP = [nil, false, true, :symbol, 1, 2.3, method(:puts)]
     ALLOW_DUP = ["1", Object.new, /foo/, [], {}, Time.now, Class.new, Module.new, BigDecimal.new("4.56")]
+  end
 
   def test_duplicable
     rubinius_skip "* Method#dup is allowed at the moment on Rubinius\n" \