[Fix rubocop#9934] Fix configuration loading to not raise an error fo…

…r an obsolete ruby version that is subsequently overridden.

changelog/fix_fix_configuration_loading_to_not_raise.md

@@ -0,0 +1 @@
+* [#9934](https://github.com/rubocop/rubocop/issues/9934): Fix configuration loading to not raise an error for an obsolete ruby version that is subsequently overridden. ([@dvandersluis][])

lib/rubocop/config.rb

@@ -51,6 +51,11 @@ def check
       self
     end
 
+    def validate_after_resolution
+      @validator.validate_after_resolution
+      self
+    end
+
     def_delegators :@hash, :[], :[]=, :delete, :dig, :each, :key?, :keys, :each_key,
                    :fetch, :map, :merge, :replace, :to_h, :to_hash, :transform_values
     def_delegators :@validator, :validate, :target_ruby_version

lib/rubocop/config_loader.rb

@@ -101,6 +101,8 @@ def configuration_from_file(config_file, check: true)
         return default_configuration if config_file == DEFAULT_FILE
 
         config = load_file(config_file, check: check)
+        config.validate_after_resolution if check
+
         if ignore_parent_exclusion?
           print 'Ignoring AllCops/Exclude from parent folders' if debug?
         else

lib/rubocop/config_validator.rb

@@ -44,14 +44,22 @@ def validate
       check_obsoletions
 
       alert_about_unrecognized_cops(invalid_cop_names)
-      check_target_ruby
       validate_new_cops_parameter
       validate_parameter_names(valid_cop_names)
       validate_enforced_styles(valid_cop_names)
       validate_syntax_cop
       reject_mutually_exclusive_defaults
     end
 
+    # Validations that should only be run after all config resolving has
+    # taken place:
+    # * The target ruby version is only checked once the entire inheritance
+    # chain has been loaded so that only the final value is validated, and
+    # any obsolete but overridden values are ignored.
+    def validate_after_resolution
+      check_target_ruby
+    end
+
     def target_ruby_version
       target_ruby.version
     end

spec/rubocop/config_loader_spec.rb

@@ -1155,6 +1155,85 @@ class Loop < Cop
       end
     end
 
+    context 'when a file inherits a configuration that specifies TargetRubyVersion' do
+      let(:file_path) { '.rubocop.yml' }
+      let(:target_ruby_version) { configuration_from_file['AllCops']['TargetRubyVersion'] }
+      let(:default_ruby_version) { RuboCop::TargetRuby::DEFAULT_VERSION }
+
+      before do
+        create_file('.rubocop-parent.yml', <<~YAML)
+          AllCops:
+            TargetRubyVersion: #{inherited_version}
+        YAML
+      end
+
+      context 'when the specified version is current' do
+        before do
+          create_file(file_path, <<~YAML)
+            inherit_from: .rubocop-parent.yml
+          YAML
+        end
+
+        let(:inherited_version) { default_ruby_version }
+
+        it 'sets TargetRubyVersion' do
+          expect(target_ruby_version).to eq(inherited_version)
+        end
+      end
+
+      context 'when the specified version is obsolete' do
+        let(:inherited_version) { '2.4' }
+
+        context 'and it is not overridden' do
+          before do
+            create_file(file_path, <<~YAML)
+              inherit_from: .rubocop-parent.yml
+            YAML
+          end
+
+          it 'raises a validation error' do
+            expect { configuration_from_file }.to raise_error(RuboCop::ValidationError) do |error|
+              expect(error.message).to start_with('RuboCop found unsupported Ruby version 2.4')
+            end
+          end
+        end
+
+        context 'and it is overridden' do
+          before do
+            create_file(file_path, <<~YAML)
+              inherit_from: .rubocop-parent.yml
+
+              AllCops:
+                TargetRubyVersion: #{default_ruby_version}
+            YAML
+          end
+
+          it 'uses the given version' do
+            expect(target_ruby_version).to eq(default_ruby_version)
+          end
+        end
+
+        context 'with deeper nesting' do
+          before do
+            create_file('.rubocop-child.yml', <<~YAML)
+              inherit_from: .rubocop-parent.yml
+            YAML
+
+            create_file('.rubocop.yml', <<~YAML)
+              inherit_from: .rubocop-child.yml
+
+              AllCops:
+                TargetRubyVersion: #{default_ruby_version}
+            YAML
+          end
+
+          it 'uses the given version' do
+            expect(target_ruby_version).to eq(default_ruby_version)
+          end
+        end
+      end
+    end
+
     context 'EnabledByDefault / DisabledByDefault' do
       def cop_enabled?(cop_class)
         configuration_from_file.for_cop(cop_class).fetch('Enabled')