prevent Dependencies#remove_const from autoloading parents [fixes #8301]

activesupport/lib/active_support/dependencies.rb

@@ -646,9 +646,22 @@ module ActiveSupport #:nodoc:
 
       constants = normalized.split('::')
       to_remove = constants.pop
-      parent_name = constants.empty? ? 'Object' : constants.join('::')
 
-      if parent = safe_constantize(parent_name)
+      if constants.empty?
+        parent = Object
+      else
+        # This method is robust to non-reachable constants.
+        #
+        # Non-reachable constants may be passed if some of the parents were
+        # autoloaded and already removed. It is easier to do a sanity check
+        # here than require the caller to be clever. We check the parent
+        # rather than the very const argument because we do not want to
+        # trigger Kernel#autoloads, see the comment below.
+        parent_name = constants.join('::')
+        return unless qualified_const_defined?(parent_name)
+        parent = constantize(parent_name)
+      end
+
       log "removing constant #{const}"
 
       # In an autoloaded user.rb like this
@@ -684,7 +697,6 @@ module ActiveSupport #:nodoc:
         log "the constant #{const} is not reachable anymore, skipping"
       end
     end
-    end
 
     protected
       def log_call(*args)

activesupport/test/dependencies_test.rb

@@ -938,6 +938,16 @@ class DependenciesTest < ActiveSupport::TestCase
     assert !defined?(ShouldNotBeAutoloaded)
   end
 
+  def test_remove_constant_does_not_autoload_already_removed_parents_as_a_side_effect
+    with_autoloading_fixtures do
+      ::A
+      ::A::B
+      ActiveSupport::Dependencies.remove_constant('A')
+      ActiveSupport::Dependencies.remove_constant('A::B')
+      assert !defined?(A)
+    end
+  end
+
   def test_load_once_constants_should_not_be_unloaded
     with_autoloading_fixtures do
       ActiveSupport::Dependencies.autoload_once_paths = ActiveSupport::Dependencies.autoload_paths