Only parse target Ruby from gemspec if array elements are strings

changelog/fix_only_parse_target_ruby_from_gemspec_if_array.md

@@ -0,0 +1 @@
+* [#12756](https://github.com/rubocop/rubocop/pull/12756): Only parse target Ruby from gemspec if array elements are strings. ([@davidrunger][])

docs/modules/ROOT/pages/configuration.adoc

@@ -647,11 +647,33 @@ AllCops:
 
 NOTE: When `ParserEngine: parser_prism` is specified, the values that can be assigned to `TargetRubyVersion` must be `3.3` or higher.
 
-Otherwise, RuboCop will then check your project for a series of files where
-the version may be specified already. The files that will be looked for are
+If a `TargetRubyVersion` is not specified in your config, then RuboCop will
+check your project for a series of other files where the Ruby version may be
+specified already. The files that will be checked are (in this order):
 `*.gemspec`, `.ruby-version`, `.tool-versions`, and `Gemfile.lock`.
-If Gemspec file has an array for `required_ruby_version`, the lowest version will be used.
-If none of the files are found a default version value will be used.
+
+If a target Ruby version cannot be found via any of the above sources, then a
+default target Ruby version will be used.
+
+=== Finding target Ruby in a `*.gemspec` file
+
+In order for RuboCop to parse a `*.gemspec` file's `required_ruby_version`, the
+Ruby version must be specified using one of these syntaxes:
+
+1. a string range, e.g. `'~> 3.2.0'` or `'>= 3.2.2'`
+2. an array of strings, e.g. `['>= 3.0.0', '< 3.4.0']`
+3. a `Gem::Requirement`, e.g. `Gem::Requirement.new('>= 3.1.2')`
+
+If a `*.gemspec` file specifies a range of supported Ruby versions via any of
+these means, then the greater of the following Ruby versions will be used:
+
+- the lowest Ruby version that is compatible with your specified range
+- the lowest version of Ruby that is still supported by your version of RuboCop
+
+If a `*.gemspec` file defines its `required_ruby_version` dynamically (e.g. by
+reading from a `.ruby-version` file, via an environment variable, referencing a
+constant or local variable, etc), then RuboCop will _not_ detect that Ruby
+version, and will instead try to find a target Ruby version elsewhere.
 
 == Setting the parser engine
 

lib/rubocop/target_ruby.rb

@@ -105,7 +105,7 @@ def version_from_gemspec_file(file)
       def version_from_right_hand_side(right_hand_side)
         gem_requirement_versions = gem_requirement_versions(right_hand_side)
 
-        if right_hand_side.array_type?
+        if right_hand_side.array_type? && right_hand_side.children.all?(&:str_type?)
           version_from_array(right_hand_side)
         elsif gem_requirement_versions
           gem_requirement_versions.map(&:value)

spec/rubocop/target_ruby_spec.rb

@@ -188,10 +188,25 @@
 
       context 'when file reads the required_ruby_version from another file' do
         it 'uses the default target ruby version' do
-          content = <<~HEREDOC
+          content = <<~'HEREDOC'
+            Gem::Specification.new do |s|
+              s.name = 'test'
+              s.required_ruby_version = Gem::Requirement.new(">= #{File.read('.ruby-version').rstrip}")
+              s.licenses = ['MIT']
+            end
+          HEREDOC
+
+          create_file(gemspec_file_path, content)
+          expect(target_ruby.version).to eq default_version
+        end
+      end
+
+      context 'when file reads the required_ruby_version from another file in an array' do
+        it 'uses the default target ruby version' do
+          content = <<~'HEREDOC'
             Gem::Specification.new do |s|
               s.name = 'test'
-              s.required_ruby_version = Gem::Requirement.new(">= \#{File.read('.ruby-version').rstrip}")
+              s.required_ruby_version = [">= #{File.read('.ruby-version').rstrip}"]
               s.licenses = ['MIT']
             end
           HEREDOC