diff --git a/spec/compiler/codegen/is_a_spec.cr b/spec/compiler/codegen/is_a_spec.cr
index cf78d6119863..e2e9deb89605 100644
--- a/spec/compiler/codegen/is_a_spec.cr
+++ b/spec/compiler/codegen/is_a_spec.cr
@@ -905,4 +905,21 @@ describe "Codegen: is_a?" do
       end
     )).to_i.should eq(2)
   end
+
+  it "does is_a? for generic type against generic class instance type (#12304)" do
+    run(%(
+      require "prelude"
+
+      class A
+      end
+
+      class B(T) < A
+      end
+
+      a = B(Int32).new.as(A)
+      b = a.as(B)
+
+      b.is_a?(B(Int32))
+    )).to_b.should be_true
+  end
 end
diff --git a/src/compiler/crystal/semantic/type_intersect.cr b/src/compiler/crystal/semantic/type_intersect.cr
index 8e533bd29b44..500c214a2cfb 100644
--- a/src/compiler/crystal/semantic/type_intersect.cr
+++ b/src/compiler/crystal/semantic/type_intersect.cr
@@ -72,6 +72,10 @@ module Crystal
       common_descendent_instance_and_generic(type1, type2)
     end
 
+    def self.common_descendent(type1 : GenericClassType, type2 : GenericClassInstanceType)
+      common_descendent(type2, type1)
+    end
+
     def self.common_descendent(type1 : GenericInstanceType, type2 : GenericInstanceType)
       common_descendent_generic_instances(type1, type2) ||
         common_descendent_base(type1, type2)