Fix Symbol#duplicable? for Ruby 2.4.0.

Ruby 2.4.0 has trouble duplicating certain symbols created from
strings via `to_sym`.

It didn't happen with `'symbol'.to_sym.dup` for some reason, but
works fine with the longer string sample.

Once a newer Ruby version with a fix is released we'll get have
a failing test case we can fix.

Ref: #27532

2 files changed, 4 insertions, 3 deletions

diff --git a/activesupport/lib/active_support/core_ext/object/duplicable.rb b/activesupport/lib/active_support/core_ext/object/duplicable.rb
index c671d34673..9485e74816 100644
--- a/activesupport/lib/active_support/core_ext/object/duplicable.rb
+++ b/activesupport/lib/active_support/core_ext/object/duplicable.rb
@@ -73,7 +73,8 @@ end
 
 class Symbol
   begin
-    :symbol.dup
+    :symbol.dup # Ruby 2.4.x.
+    'symbol_from_string'.to_sym.dup # Some symbols can't `dup` in Ruby 2.4.0.
   rescue TypeError
 
     # Symbols are not duplicable:
diff --git a/activesupport/test/core_ext/object/duplicable_test.rb b/activesupport/test/core_ext/object/duplicable_test.rb
index f78a5f8496..fb140a5b76 100644
--- a/activesupport/test/core_ext/object/duplicable_test.rb
+++ b/activesupport/test/core_ext/object/duplicable_test.rb
@@ -5,8 +5,8 @@ require "active_support/core_ext/numeric/time"
 
 class DuplicableTest < ActiveSupport::TestCase
   if RUBY_VERSION >= "2.4.0"
-    RAISE_DUP = [method(:puts), Complex(1), Rational(1)]
-    ALLOW_DUP = ["1", Object.new, /foo/, [], {}, Time.now, Class.new, Module.new, BigDecimal.new("4.56"), nil, false, true, :symbol, 1, 2.3]
+    RAISE_DUP = [method(:puts), Complex(1), Rational(1), 'symbol_from_string'.to_sym]
+    ALLOW_DUP = ["1", Object.new, /foo/, [], {}, Time.now, Class.new, Module.new, BigDecimal.new("4.56"), nil, false, true, 1, 2.3]
   else
     RAISE_DUP = [nil, false, true, :symbol, 1, 2.3, method(:puts), Complex(1), Rational(1)]
     ALLOW_DUP = ["1", Object.new, /foo/, [], {}, Time.now, Class.new, Module.new, BigDecimal.new("4.56")]