rb_ivar_defined: handle complex modules

It was assuming only objects can be complex.

test/ruby/test_shapes.rb

@@ -345,6 +345,45 @@ class TestShapes < Test::Unit::TestCase
     end;
   end
 
+  def test_run_out_of_shape_instance_variable_defined
+    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      class A
+        attr_reader :a, :b, :c, :d
+        def initialize
+          @a = @b = @c = @d = 1
+        end
+      end
+
+      o = Object.new
+      i = 0
+      while RubyVM::Shape.shapes_available > 0
+        o.instance_variable_set(:"@i#{i}", 1)
+        i += 1
+      end
+
+      a = A.new
+      assert_equal true, a.instance_variable_defined?(:@a)
+    end;
+  end
+
+  def test_run_out_of_shape_instance_variable_defined_on_module
+    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      o = Object.new
+      i = 0
+      while RubyVM::Shape.shapes_available > 0
+        o.instance_variable_set(:"@i#{i}", 1)
+        i += 1
+      end
+
+      module A
+        @a = @b = @c = @d = 1
+      end
+
+      assert_equal true, A.instance_variable_defined?(:@a)
+    end;
+  end
   def test_run_out_of_shape_remove_instance_variable
     assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
     begin;

variable.c

@@ -1809,7 +1809,27 @@ rb_ivar_defined(VALUE obj, ID id)
     if (SPECIAL_CONST_P(obj)) return Qfalse;
     if (rb_shape_obj_too_complex(obj)) {
         VALUE idx;
-        if (!rb_st_lookup(ROBJECT_IV_HASH(obj), id, &idx)) {
+        st_table *table = NULL;
+        switch (BUILTIN_TYPE(obj)) {
+          case T_CLASS:
+          case T_MODULE:
+            table = (st_table *)RCLASS_IVPTR(obj);
+            break;
+
+          case T_OBJECT:
+            table = ROBJECT_IV_HASH(obj);
+            break;
+
+          default: {
+            struct gen_ivtbl *ivtbl;
+            if (rb_gen_ivtbl_get(obj, 0, &ivtbl)) {
+                table = ivtbl->as.complex.table;
+            }
+            break;
+          }
+        }
+
+        if (!table || !rb_st_lookup(table, id, &idx)) {
             return Qfalse;
         }