Merge pull request #24517 from estolfo/transform-keys-return-type-master

Restore Hash#transform_keys behavior to always return a Hash instance

activesupport/lib/active_support/core_ext/hash/keys.rb

@@ -11,7 +11,7 @@ class Hash
   #  hash.transform_keys.with_index { |k, i| [k, i].join } # => {"name0"=>"Rob", "age1"=>"28"}
   def transform_keys
     return enum_for(:transform_keys) { size } unless block_given?
-    result = self.class.new
+    result = {}
     each_key do |key|
       result[yield(key)] = self[key]
     end

activesupport/test/core_ext/hash/transform_keys_test.rb

@@ -43,4 +43,20 @@ class TransformKeysTest < ActiveSupport::TestCase
     original.transform_keys!.with_index { |k, i| [k, i].join.to_sym }
     assert_equal({ a0: 'a', b1: 'b' }, original)
   end
+
+  test "transform_keys returns a Hash instance when self is inherited from Hash" do
+    class HashDescendant < ::Hash
+      def initialize(elements = nil)
+        super(elements)
+        (elements || {}).each_pair{ |key, value| self[key] = value }
+      end
+    end
+
+    original = HashDescendant.new({ a: 'a', b: 'b' })
+    mapped = original.transform_keys { |k| "#{k}!".to_sym }
+
+    assert_equal({ a: 'a', b: 'b' }, original)
+    assert_equal({ a!: 'a', b!: 'b' }, mapped)
+    assert_equal(::Hash, mapped.class)
+  end
 end