Fix required_ruby_version issue when using Gem::Requirement (#9037)

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 #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

@@ -123,6 +123,10 @@ class GemspecFile < Source
         (send _ :required_ruby_version= $_)
       PATTERN
 
+      def_node_matcher :gem_requirement?, <<~PATTERN
+        (send (const(const _ :Gem):Requirement) :new $str)
+      PATTERN
+
       def name
         "`required_ruby_version` parameter (in #{gemspec_filename})"
       end
@@ -136,10 +140,9 @@ def find_version
         version = version_from_gemspec_file(file)
         return if version.nil?
 
-        if version.array_type?
-          versions = version.children.map { |v| version_from_str(v.str_content) }
-          return versions.compact.min
-        end
+        requirement = version.children.last
+        return version_from_array(version) if version.array_type?
+        return version_from_array(requirement) if gem_requirement? version
 
         version_from_str(version.str_content)
       end
@@ -161,6 +164,11 @@ def version_from_gemspec_file(file)
         required_ruby_version(processed_source.ast).first
       end
 
+      def version_from_array(array)
+        versions = array.children.map { |v| version_from_str(v.is_a?(String) ? v : v.str_content) }
+        versions.compact.min
+      end
+
       def version_from_str(str)
         str.match(/^(?:>=|<=)?\s*(?<version>\d+(?:\.\d+)*)/) do |md|
           md[:version].to_f

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') }