Use actual class instead of singleton class in frozen error message

With the following code:

```ruby
object = []
object.singleton_class
object.freeze
object.instance_variable_set(:@a, 42)
```

The previous error message was:

```
can't modify frozen #<Class:#<Array:0x00000631d1308f78>>: []
```

With this change, the error message is:

```
can't modify frozen Array: []
```

Since we show the inspect value of the affected object, I think
including the singleton class instead of the actual class if it
exists makes the error message harder to understand.

2 files changed, 17 insertions, 1 deletions

diff --git a/test/ruby/test_frozen.rb b/test/ruby/test_frozen.rb
index 2918a2afd8..6721cb1128 100644
--- a/test/ruby/test_frozen.rb
+++ b/test/ruby/test_frozen.rb
@@ -27,4 +27,20 @@ class TestFrozen < Test::Unit::TestCase
     str.freeze
     assert_raise(FrozenError) { str.instance_variable_set(:@b, 1) }
   end
+
+  def test_setting_ivar_on_frozen_string_with_singleton_class
+    str = "str"
+    str.singleton_class
+    str.freeze
+    assert_raise_with_message(FrozenError, "can't modify frozen String: \"str\"") { str.instance_variable_set(:@a, 1) }
+  end
+
+  class A
+    freeze
+  end
+
+  def test_setting_ivar_on_frozen_class
+    assert_raise_with_message(FrozenError, "can't modify frozen Class: TestFrozen::A") { A.instance_variable_set(:@a, 1) }
+    assert_raise_with_message(FrozenError, "can't modify frozen Class: #<Class:TestFrozen::A>") { A.singleton_class.instance_variable_set(:@a, 1) }
+  end
 end
diff --git a/test/ruby/test_zjit.rb b/test/ruby/test_zjit.rb
index 81d9404070..49b3616425 100644
--- a/test/ruby/test_zjit.rb
+++ b/test/ruby/test_zjit.rb
@@ -2057,7 +2057,7 @@ class TestZJIT < Test::Unit::TestCase
   end
 
   def test_setclassvariable_raises
-    assert_compiles '"can\'t modify frozen #<Class:Foo>: Foo"', %q{
+    assert_compiles '"can\'t modify frozen Class: Foo"', %q{
       class Foo
         def self.test = @@x = 42
         freeze