Warn on empty or open required_ruby_version specification attribute.

lib/rubygems/specification_policy.rb

@@ -103,6 +103,8 @@ def validate_optional(strict)
 
     validate_dependencies
 
+    validate_required_ruby_version
+
     validate_extensions
 
     validate_removed_attributes
@@ -227,6 +229,12 @@ def validate_dependencies # :nodoc:
     end
   end
 
+  def validate_required_ruby_version
+    if @specification.required_ruby_version.requirements == [Gem::Requirement::DefaultRequirement]
+      warning "make sure you specify the oldest ruby version constraint (like \">= 3.0\") that you want your gem to support by setting the `required_ruby_version` gemspec attribute"
+    end
+  end
+
   ##
   # Issues a warning for each file to be packaged which is world-readable.
   #

test/rubygems/specifications/rubyforge-0.0.1.gemspec

@@ -1,14 +1,15 @@
 # frozen_string_literal: true
 
 Gem::Specification.new do |s|
-  s.name              = "rubyforge"
-  s.version           = "0.0.1"
-  s.platform          = "ruby"
-  s.require_paths     = ["lib"]
-  s.summary           = "A very bar gem"
-  s.authors           = ["unknown"]
-  s.license           = "MIT"
-  s.homepage          = "http://example.com"
-  s.files             = ["README.md"]
-  s.rubyforge_project = "abc"
+  s.name                  = "rubyforge"
+  s.version               = "0.0.1"
+  s.platform              = "ruby"
+  s.require_paths         = ["lib"]
+  s.summary               = "A very bar gem"
+  s.authors               = ["unknown"]
+  s.license               = "MIT"
+  s.homepage              = "http://example.com"
+  s.files                 = ["README.md"]
+  s.rubyforge_project     = "abc"
+  s.required_ruby_version = ">= 1.9.3"
 end

test/rubygems/test_gem_commands_build_command.rb

@@ -28,6 +28,7 @@ def setup
     @gem = util_spec "some_gem" do |s|
       s.license = "AGPL-3.0-only"
       s.files = ["README.md"]
+      s.required_ruby_version = "2.3.0"
     end
 
     @cmd = Gem::Commands::BuildCommand.new
@@ -178,6 +179,7 @@ def test_execute_rubyforge_project_warning
   def test_execute_strict_with_warnings
     bad_gem = util_spec "some_bad_gem" do |s|
       s.files = ["README.md"]
+      s.required_ruby_version = ">= 1.9.3"
     end
 
     gemspec_file = File.join(@tempdir, bad_gem.spec_name)

test/rubygems/test_gem_specification.rb

@@ -90,6 +90,7 @@ def setup
     Gem.instance_variable_set(:'@default_source_date_epoch', nil)
 
     @a1 = util_spec "a", "1" do |s|
+      s.required_ruby_version = ">= 2.3.0"
       s.executable = "exec"
       s.test_file = "test/suite.rb"
       s.requirements << "A working computer"
@@ -2707,6 +2708,53 @@ def test_validate_dependencies_allowed_duplicates
     end
   end
 
+  def test_validate_no_required_ruby_versions
+    util_setup_validate
+
+    Dir.chdir @tempdir do
+      use_ui @ui do
+        @a1.required_ruby_version = nil # reset
+        @a1.validate
+      end
+
+      assert_equal <<-EXPECTED, @ui.error
+#{w}:  make sure you specify the oldest ruby version constraint (like \">= 3.0\") that you want your gem to support by setting the `required_ruby_version` gemspec attribute
+#{w}:  See https://guides.rubygems.org/specification-reference/ for help
+      EXPECTED
+    end
+  end
+
+  def test_validate_open_required_ruby_versions
+    util_setup_validate
+
+    Dir.chdir @tempdir do
+      @a1.required_ruby_version = ">= 0"
+
+      use_ui @ui do
+        @a1.validate
+      end
+
+      assert_equal <<-EXPECTED, @ui.error
+#{w}:  make sure you specify the oldest ruby version constraint (like \">= 3.0\") that you want your gem to support by setting the `required_ruby_version` gemspec attribute
+#{w}:  See https://guides.rubygems.org/specification-reference/ for help
+      EXPECTED
+    end
+  end
+
+  def test_validate_valid_required_ruby_versions
+    util_setup_validate
+
+    Dir.chdir @tempdir do
+      @a1.required_ruby_version = ">= 2.3.0"
+
+      use_ui @ui do
+        @a1.validate
+      end
+
+      assert_equal "", @ui.error, "warning"
+    end
+  end
+
   def test_validate_prerelease_dependencies_with_prerelease_version
     util_setup_validate
 
@@ -3683,6 +3731,7 @@ def test_metadata_link_validation_warns_for_duplicates
 
     Dir.chdir @tempdir do
       @m2 = quick_gem "m", "2" do |s|
+        s.required_ruby_version = ">= 2.3.0"
         s.files = %w[lib/code.rb]
         s.licenses = "BSD-2-Clause"
         s.metadata = {