Add chacterization test for method_added hooks

spec/misc/how_method_added_hooks_work_spec.rb

@@ -0,0 +1,63 @@
+# frozen_string_literal: true
+
+require "spec_helper"
+
+module TypeToolkit
+  module Misc
+    # This test just characterizes how `method_added` hooks work. It doesn't have anything to do with TypeToolkit itself,
+    # but it's useful for devs to confirm how this works across instance and class methods.
+    class HowMethodAddedHooksWorkSpec < Minitest::Spec
+      it "characterizes how method_added hooks work" do
+        test_context = self
+
+        Class.new do
+          class << self
+            attr_reader :__last_method_def
+
+            class << self
+              def __last_method_def
+                attached_object.__last_method_def
+              end
+            end
+
+            def method_added(m)
+              is_singleton_method = false
+              @__last_method_def = [m, is_singleton_method]
+
+              super
+            end
+
+            def singleton_method_added(m)
+              return super if m == :singleton_method_added
+
+              is_singleton_method = true
+              @__last_method_def = [m, is_singleton_method]
+
+              super
+            end
+          end
+
+          test_context.assert_nil __last_method_def
+
+          def instance_method; end
+
+          test_context.assert_equal [:instance_method, false], __last_method_def
+
+          def self.singleton_method_via_self_dot; end # rubocop:disable Style/ClassMethodsDefinitions
+
+          test_context.assert_equal [:singleton_method_via_self_dot, true], __last_method_def
+
+          r = class << self
+                def singleton_method_via_sclass; end
+
+                __last_method_def
+          end
+
+          test_context.assert_equal [:singleton_method_via_sclass, true], r
+
+          test_context.assert_equal [:singleton_method_via_sclass, true], __last_method_def
+        end
+      end
+    end
+  end
+end