Skip ActiveSupport deprecation proxies when gathering DSL constants

lib/tapioca/dsl/compiler.rb

@@ -75,7 +75,19 @@ def all_modules
             @@requested_constants.grep(Module)
           else
             ObjectSpace.each_object(Module).to_a
-          end.freeze #: Enumerable[Module[top]]?
+          end.reject { |mod| deprecated_constant_proxy?(mod) }.freeze #: Enumerable[Module[top]]?
+        end
+
+        # Rails 8.1+ wraps deprecated constants in DeprecatedConstantProxy, which
+        # undefines most instance methods and warns from method_missing. Inspecting
+        # those modules during DSL discovery (is_a?, singleton_class, etc.) emits
+        # deprecation warnings even though Tapioca is only enumerating ObjectSpace.
+        # class_of uses Kernel#class so we can recognize the proxy without warning.
+        #: (Module[top] mod) -> bool
+        def deprecated_constant_proxy?(mod)
+          proxy_class_name = name_of(class_of(mod))
+          proxy_class_name == "ActiveSupport::Deprecation::DeprecatedConstantProxy" ||
+            proxy_class_name == "ActiveSupport::DeprecatedConstantProxy"
         end
       end
 

spec/tapioca/dsl/compilers/active_support_concern_spec.rb

@@ -14,6 +14,23 @@ def before_setup
           end
 
           describe "gather_constants" do
+            it "does not gather ActiveSupport deprecation proxies and does not warn" do
+              require "active_support/concurrency/load_interlock_aware_monitor"
+
+              warnings = []
+              previous_behavior = ActiveSupport.deprecator.behavior
+              ActiveSupport.deprecator.behavior = ->(message, *) { warnings << message.to_s }
+
+              begin
+                constants = gathered_constants
+              ensure
+                ActiveSupport.deprecator.behavior = previous_behavior
+              end
+
+              refute_includes(constants, "ActiveSupport::Concurrency::LoadInterlockAwareMonitor")
+              assert_empty(warnings.grep(/LoadInterlockAwareMonitor/))
+            end
+
             it "does not gather anonymous constants" do
               add_ruby_file("test_case.rb", <<~RUBY)
                 module TestCase