Module#remove_method: Check frozen on the right object

Previously, the frozen check happened on `RCLASS_ORIGIN(self)`, which
can return an iclass. The frozen check is supposed to respond to objects
that users can call methods on while iclasses are hidden from users.
Other mutation methods like Module#{define_method,alias_method,public}
don't do this. Check frozen status on the module itself.

Fixes [Bug #19164] and [Bug #19166].

Co-authored-by: Alan Wu <XrXr@users.noreply.github.com>

test/ruby/test_class.rb

@@ -809,4 +809,13 @@ PREP
 3_000_000.times(&code)
 CODE
   end
+
+  def test_instance_freeze_dont_freeze_the_class_bug_19164
+    klass = Class.new
+    klass.prepend(Module.new)
+
+    klass.new.freeze
+    klass.define_method(:bar) {}
+    assert_equal klass, klass.remove_method(:bar), '[Bug #19164]'
+  end
 end

test/ruby/test_module.rb

@@ -2350,6 +2350,18 @@ class TestModule < Test::Unit::TestCase
     assert_equal(:foo, removed)
   end
 
+  def test_frozen_prepend_remove_method
+    [Module, Class].each do |klass|
+      mod = klass.new do
+        prepend(Module.new)
+        def foo; end
+      end
+      mod.freeze
+      assert_raise(FrozenError, '[Bug #19166]') { mod.send(:remove_method, :foo) }
+      assert_equal([:foo], mod.instance_methods(false))
+    end
+  end
+
   def test_prepend_class_ancestors
     bug6658 = '[ruby-core:45919]'
     m = labeled_module("m")

vm_method.c

@@ -1541,8 +1541,8 @@ remove_method(VALUE klass, ID mid)
     rb_method_entry_t *me = 0;
     VALUE self = klass;
 
-    klass = RCLASS_ORIGIN(klass);
     rb_class_modify_check(klass);
+    klass = RCLASS_ORIGIN(klass);
     if (mid == object_id || mid == id__send__ || mid == idInitialize) {
         rb_warn("removing `%s' may cause serious problems", rb_id2name(mid));
     }