[Fix rubocop#6926] Allow nil values to unset config defaults

CHANGELOG.md

@@ -13,6 +13,7 @@
 * Do not annotate message with cop name in JSON output. ([@elebow][])
 * [#6914](https://github.com/rubocop-hq/rubocop/issues/6914): [Fix #6914] Fix an error for `Rails/RedundantAllowNil` when with interpolations. ([@Blue-Pix][])
 * [#6941](https://github.com/rubocop-hq/rubocop/issues/6941): Add missing absence validations to `Rails/Validation`. ([@jmanian][])
+* [#6926](https://github.com/rubocop-hq/rubocop/issues/6926): [Fix #6926] Allow nil values to unset config defaults. ([@dduugg][])
 
 ### Changes
 

lib/rubocop/config_loader_resolver.rb

@@ -71,9 +71,8 @@ def merge_with_default(config, config_file)
         config = handle_disabled_by_default(config, default_configuration)
       end
 
-      Config.new(merge(default_configuration, config,
-                       inherit_mode: config['inherit_mode'] || {}),
-                 config_file)
+      opts = { inherit_mode: config['inherit_mode'] || {}, unset_nil: true }
+      Config.new(merge(default_configuration, config, opts), config_file)
     end
 
     # Return a recursive merge of two hashes. That is, a normal hash merge,
@@ -85,7 +84,9 @@ def merge(base_hash, derived_hash, **opts)
       result = base_hash.merge(derived_hash)
       keys_appearing_in_both = base_hash.keys & derived_hash.keys
       keys_appearing_in_both.each do |key|
-        if base_hash[key].is_a?(Hash)
+        if opts[:unset_nil] && derived_hash[key].nil?
+          result.delete(key)
+        elsif base_hash[key].is_a?(Hash)
           result[key] = merge(base_hash[key], derived_hash[key], **opts)
         elsif should_union?(base_hash, key, opts[:inherit_mode])
           result[key] = base_hash[key] | derived_hash[key]

spec/rubocop/config_loader_spec.rb

@@ -277,6 +277,45 @@
       end
     end
 
+    context 'when a file inherits and overrides a hash with nil' do
+      let(:file_path) { '.rubocop.yml' }
+
+      before do
+        create_file('.rubocop_parent.yml', <<-YAML.strip_indent)
+          Style/InverseMethods:
+            InverseMethods:
+              :any?: :none?
+              :even?: :odd?
+              :==: :!=
+              :=~: :!~
+              :<: :>=
+              :>: :<=
+        YAML
+
+        create_file('.rubocop.yml', <<-YAML.strip_indent)
+          inherit_from: .rubocop_parent.yml
+
+          Style/InverseMethods:
+            InverseMethods:
+              :<: ~
+              :>: ~
+              :foo: :bar
+        YAML
+      end
+
+      it 'removes hash keys with nil values' do
+        inverse_methods =
+          configuration_from_file['Style/InverseMethods']['InverseMethods']
+        expect(inverse_methods).to eq(
+          '==': :!=,
+          '=~': :!~,
+          any?: :none?,
+          even?: :odd?,
+          foo: :bar
+        )
+      end
+    end
+
     context 'when inherit_mode is set to merge for Exclude' do
       let(:file_path) { '.rubocop.yml' }