1 files changed, 12 insertions, 6 deletions
diff --git a/activesupport/lib/active_support/core_ext/module/delegation.rb b/activesupport/lib/active_support/core_ext/module/delegation.rb
index b8996ecb10..14d7f0c484 100644
--- a/activesupport/lib/active_support/core_ext/module/delegation.rb
+++ b/activesupport/lib/active_support/core_ext/module/delegation.rb
@@ -205,18 +205,18 @@ class Module
       if allow_nil
         method_def = [
           "def #{method_prefix}#{method}(#{definition})",
-          "_ = #{to}",
-          "if !_.nil? || nil.respond_to?(:#{method})",
-          "  _.#{method}(#{definition})",
-          "end",
-        "end"
+          "  _ = #{to}",
+          "  if !_.nil? || nil.respond_to?(:#{method})",
+          "    _.#{method}(#{definition})",
+          "  end",
+          "end"
         ].join ";"
       else
         exception = %(raise DelegationError, "#{self}##{method_prefix}#{method} delegated to #{to}.#{method}, but #{to} is nil: \#{self.inspect}")
 
         method_def = [
           "def #{method_prefix}#{method}(#{definition})",
-          " _ = #{to}",
+          "  _ = #{to}",
           "  _.#{method}(#{definition})",
           "rescue NoMethodError => e",
           "  if _.nil? && e.name == :#{method}",
@@ -276,6 +276,11 @@ class Module
   # The delegated method must be public on the target, otherwise it will
   # raise +DelegationError+. If you wish to instead return +nil+,
   # use the <tt>:allow_nil</tt> option.
+  #
+  # The <tt>marshal_dump</tt> and <tt>_dump</tt> methods are exempt from
+  # delegation due to possible interference when calling
+  # <tt>Marshal.dump(object)</tt>, should the delegation target method
+  # of <tt>object</tt> add or remove instance variables.
   def delegate_missing_to(target, allow_nil: nil)
     target = target.to_s
     target = "self.#{target}" if DELEGATION_RESERVED_METHOD_NAMES.include?(target)
@@ -285,6 +290,7 @@ class Module
         # It may look like an oversight, but we deliberately do not pass
         # +include_private+, because they do not get delegated.
 
+        return false if name == :marshal_dump || name == :_dump
         #{target}.respond_to?(name) || super
       end