Fix required_ruby_version issue when using Gem::Requirement

This fixes an issue when the `required_ruby_version` uses `Gem::Requirement` to define the Ruby version.

e.g.

```
s.required_ruby_version = Gem::Requirement.new('< 3.0.0')
```

Related to rubocop#8761

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### Bug fixes
+
+* [#9037](https://github.com/rubocop-hq/rubocop/pull/9037): Fix `required_ruby_version` issue when using `Gem::Requirement`. ([@cetinajero][])
+
 ## 1.3.0 (2020-11-12)
 
 ### New features

lib/rubocop/target_ruby.rb

@@ -141,9 +141,17 @@ def find_version
           return versions.compact.min
         end
 
+        return gem_requirement_version(version) if version.send_type?
+
         version_from_str(version.str_content)
       end
 
+      def gem_requirement_version(version)
+        gem_requirement = version.children.last
+        versions = gem_requirement.children.map { |v| version_from_str(v) }
+        versions.compact.min
+      end
+
       def gemspec_filename
         @gemspec_filename ||= begin
           basename = Pathname.new(@config.base_dir_for_path_parameters).basename.to_s

spec/rubocop/target_ruby_spec.rb

@@ -305,6 +305,53 @@
           end
         end
 
+        context 'when file contains `required_ruby_version` as a requirement' do
+          let(:base_path) { configuration.base_dir_for_path_parameters }
+          let(:gemspec_file_path) { File.join(base_path, 'example.gemspec') }
+
+          it 'sets target_ruby from required_ruby_version from exact requirement version' do
+            content =
+              <<-HEREDOC
+                Gem::Specification.new do |s|
+                  s.name = 'test'
+                  s.required_ruby_version = Gem::Requirement.new('2.7.4')
+                  s.licenses = ['MIT']
+                end
+              HEREDOC
+
+            create_file(gemspec_file_path, content)
+            expect(target_ruby.version).to eq 2.7
+          end
+
+          it 'sets target_ruby from required_ruby_version from inclusive requirement range' do
+            content =
+              <<-HEREDOC
+                Gem::Specification.new do |s|
+                  s.name = 'test'
+                  s.required_ruby_version = Gem::Requirement.new('>= 3.0.0')
+                  s.licenses = ['MIT']
+                end
+              HEREDOC
+
+            create_file(gemspec_file_path, content)
+            expect(target_ruby.version).to eq 3.0
+          end
+
+          it 'sets default target_ruby from exclusive requirement range' do
+            content =
+              <<-HEREDOC
+                Gem::Specification.new do |s|
+                  s.name = 'test'
+                  s.required_ruby_version = Gem::Requirement.new('< 3.0.0')
+                  s.licenses = ['MIT']
+                end
+              HEREDOC
+
+            create_file(gemspec_file_path, content)
+            expect(target_ruby.version).to eq default_version
+          end
+        end
+
         context 'when file contains `required_ruby_version` as an array' do
           let(:base_path) { configuration.base_dir_for_path_parameters }
           let(:gemspec_file_path) { File.join(base_path, 'example.gemspec') }