Fix segmentation fault when `Module#name` returns non string value [Bug #17754]

* Add test for NoMethodError#to_s does not segfault

* Ensure no segfault even if Module#name is overridden

2 files changed, 19 insertions, 1 deletions

diff --git a/error.c b/error.c
index 9da51a3a39..5dd6b2b765 100644
--- a/error.c
+++ b/error.c
@@ -1965,8 +1965,10 @@ name_err_mesg_to_str(VALUE obj)
 	    d = rb_protect(name_err_mesg_receiver_name, obj, &state);
 	    if (state || d == Qundef || d == Qnil)
 		d = rb_protect(rb_inspect, obj, &state);
-	    if (state)
+	    if (state) {
 		rb_set_errinfo(Qnil);
+	    }
+	    d = rb_check_string_type(d);
 	    if (NIL_P(d)) {
 		d = rb_any_to_s(obj);
 	    }
diff --git a/test/ruby/test_nomethod_error.rb b/test/ruby/test_nomethod_error.rb
index 170a6c6c57..8b81052905 100644
--- a/test/ruby/test_nomethod_error.rb
+++ b/test/ruby/test_nomethod_error.rb
@@ -90,4 +90,20 @@ class TestNoMethodError < Test::Unit::TestCase
       str.__send__(id)
     end
   end
+
+  def test_to_s
+    pre = Module.new do
+      def name
+        BasicObject.new
+      end
+    end
+    mod = Module.new
+    mod.singleton_class.prepend(pre)
+
+    err = assert_raise(NoMethodError) do
+      mod.this_method_does_not_exist
+    end
+
+    assert_match(/undefined method.+this_method_does_not_exist.+for.+Module/, err.to_s)
+  end
 end