Raise an error when building a gem that has a self reference:

- ### Problem

  A gem that has a self-reference in its dependencies would previously
  get a warning during `gem build`, saying it's "discouradged".
  A gem that includes a self-reference can't be updated due to bundler
  filtering it out.

  https://github.com/ruby/rubygems/blob/6fd37f4afeb3943cf508d1394fcf4338a1266f2e/bundler/lib/bundler/resolver.rb#L405-L410

  I think we should be more strict and prevent the gem from building.

  ### Solution

  Raise an explicit error.

  This codepath is only hit when running `gem build`, so this change
  won't affect existing consumers of those gems (it was previously
  possible to install those gems, but not update them, see
  #9346 for more context).

Author: Edouard-chin

lib/rubygems/specification_policy.rb

@@ -192,15 +192,14 @@ def validate_duplicate_dependencies # :nodoc:
   # Checks that the gem does not depend on itself.
 
   def validate_dependencies # :nodoc:
-    warning_messages = []
+    error_messages = []
     @specification.dependencies.each do |dep|
-      if dep.name == @specification.name # warn on self reference
-        warning_messages << "Self referencing dependency is unnecessary and strongly discouraged."
+      if dep.name == @specification.name # error on self reference
+        error_messages << "Dependencies of this gem include a self-reference."
       end
     end
-    if warning_messages.any?
-      warning_messages.each {|warning_message| warning warning_message }
-    end
+
+    error error_messages.join if error_messages.any?
   end
 
   def validate_required_ruby_version

test/rubygems/test_gem_specification.rb

@@ -2812,14 +2812,13 @@ def test_validate_self_referencing_dependencies
     Dir.chdir @tempdir do
       @a1.add_dependency @a1.name, "1"
 
-      use_ui @ui do
+      e = assert_raise Gem::InvalidSpecificationException do
         @a1.validate
       end
 
-      assert_equal <<-EXPECTED, @ui.error
-#{w}:  Self referencing dependency is unnecessary and strongly discouraged.
-#{w}:  See https://guides.rubygems.org/specification-reference/ for help
-      EXPECTED
+      expected = "Dependencies of this gem include a self-reference."
+
+      assert_equal expected, e.message
     end
   end