Avoid triggering autoload in Module#const_defined?(String)

[Bug #15780]

object.c

@@ -2694,16 +2694,19 @@ rb_mod_const_defined(int argc, VALUE *argv, VALUE mod)
         if (!RTEST(recur)) {
 	    if (!rb_const_defined_at(mod, id))
 		return Qfalse;
+            if (p == pend) return Qtrue;
 	    mod = rb_const_get_at(mod, id);
 	}
         else if (beglen == 0) {
             if (!rb_const_defined(mod, id))
                 return Qfalse;
+            if (p == pend) return Qtrue;
             mod = rb_const_get(mod, id);
         }
         else {
             if (!rb_const_defined_from(mod, id))
                 return Qfalse;
+            if (p == pend) return Qtrue;
             mod = rb_const_get_from(mod, id);
         }
 #endif

spec/ruby/core/module/autoload_spec.rb

@@ -449,6 +449,7 @@ describe "Module#autoload" do
   it "does not load the file when accessing the constants table of the module" do
     ModuleSpecs::Autoload.autoload :P, @non_existent
     ModuleSpecs::Autoload.const_defined?(:P).should be_true
+    ModuleSpecs::Autoload.const_defined?("P").should be_true
   end
 
   it "loads the file when opening a module that is the autoloaded constant" do

test/ruby/test_autoload.rb

@@ -42,8 +42,10 @@ p Foo::Bar
     require 'tmpdir'
     Dir.mktmpdir('autoload') {|tmpdir|
       tmpfile = tmpdir + '/foo.rb'
+      tmpfile2 = tmpdir + '/bar.rb'
       a = Module.new do
         autoload :X, tmpfile
+        autoload :Y, tmpfile2
       end
       b = Module.new do
         include a
@@ -52,6 +54,10 @@ p Foo::Bar
       assert_equal(true, b.const_defined?(:X))
       assert_equal(tmpfile, a.autoload?(:X), bug4565)
       assert_equal(tmpfile, b.autoload?(:X), bug4565)
+      assert_equal(true, a.const_defined?("Y"))
+      assert_equal(true, b.const_defined?("Y"))
+      assert_equal(tmpfile2, a.autoload?("Y"))
+      assert_equal(tmpfile2, b.autoload?("Y"))
     }
   end