Revert "Revert "[rubygems/rubygems] [Require] Ensure -I beats a default gem""

This reverts commit 75d29db8f9.

lib/rubygems/core_ext/kernel_require.rb

@@ -36,6 +36,26 @@ module Kernel
 
     path = path.to_path if path.respond_to? :to_path
 
+    resolved_path = begin
+      rp = nil
+      $LOAD_PATH[0...Gem.load_path_insert_index || -1].each do |lp|
+        Gem.suffixes.each do |s|
+          full_path = File.expand_path(File.join(lp, "#{path}#{s}").untaint)
+          if File.file?(full_path)
+            rp = full_path
+            break
+          end
+        end
+        break if rp
+      end
+      rp
+    end
+
+    if resolved_path
+      RUBYGEMS_ACTIVATION_MONITOR.exit
+      return gem_original_require(resolved_path)
+    end
+
     if spec = Gem.find_unresolved_default_spec(path)
       begin
         Kernel.send(:gem, spec.name, "#{Gem::Requirement.default}.a")

lib/rubygems/test_case.rb

@@ -739,6 +739,7 @@ class Gem::TestCase < (defined?(Minitest::Test) ? Minitest::Test : MiniTest::Uni
     spec.files = files
 
     lib_dir = File.join(@tempdir, "default_gems", "lib")
+    lib_dir.instance_variable_set(:@gem_prelude_index, lib_dir)
     $LOAD_PATH.unshift(lib_dir)
     files.each do |file|
       rb_path = File.join(lib_dir, file)

test/rubygems/test_require.rb

@@ -87,6 +87,39 @@ class TestGemRequire < Gem::TestCase
     end
   end
 
+  # Providing -I on the commandline should always beat gems
+  def test_dash_i_beats_default_gems
+    a1 = new_default_spec "a", "1", {"b" => "= 1"}, "test_gem_require_a.rb"
+    b1 = new_default_spec "b", "1", {"c" => "> 0"}, "b/c.rb"
+    c1 = new_default_spec "c", "1", nil, "c/c.rb"
+    c2 = new_default_spec "c", "2", nil, "c/c.rb"
+
+    install_default_specs c1, c2, b1, a1
+
+    dir = Dir.mktmpdir("test_require", @tempdir)
+    dash_i_arg = File.join dir, 'lib'
+
+    c_rb = File.join dash_i_arg, 'c', 'c.rb'
+
+    FileUtils.mkdir_p File.dirname c_rb
+    File.open(c_rb, 'w') { |f| f.write "class Object; HELLO = 'world' end" }
+
+    assert_require 'test_gem_require_a'
+
+    lp = $LOAD_PATH.dup
+
+    # Pretend to provide a commandline argument that overrides a file in gem b
+    $LOAD_PATH.unshift dash_i_arg
+
+    assert_require 'b/c'
+    assert_require 'c/c' # this should be required from -I
+    assert_equal "world", ::Object::HELLO
+    assert_equal %w(a-1 b-1), loaded_spec_names
+  ensure
+    $LOAD_PATH.replace lp
+    Object.send :remove_const, :HELLO if Object.const_defined? :HELLO
+  end
+
   def test_concurrent_require
     Object.const_set :FILE_ENTERED_LATCH, Latch.new(2)
     Object.const_set :FILE_EXIT_LATCH, Latch.new(1)