Fix .inhertited infinite recursion bug

Author: amomchilov

lib/tapioca/runtime/generic_type_registry.rb

@@ -117,7 +117,7 @@ def create_generic_type(constant, name)
             # the generic class `Foo[Bar]` is still a `Foo`. That is:
             # `Foo[Bar].new.is_a?(Foo)` should be true, which isn't the case
             # if we just clone the class. But subclassing works just fine.
-            create_safe_subclass(constant)
+            create_safe_subclass(constant, name)
           else
             # This can only be a module and it is fine to just clone modules
             # since they can't have instances and will not have `is_a?` relationships.
@@ -151,31 +151,32 @@ def create_generic_type(constant, name)
           generic_type
         end
 
-        sig { params(constant: T::Class[T.anything]).returns(T::Class[T.anything]) }
-        def create_safe_subclass(constant)
+        sig { params(constant: T::Class[T.anything], name: String).returns(T::Class[T.anything]) }
+        def create_safe_subclass(constant, name)
           # Lookup the "inherited" class method
           inherited_method = constant.method(:inherited)
           # and the module that defines it
           owner = inherited_method.owner
 
-          # If no one has overriden the inherited method yet, just subclass
-          return Class.new(constant) if Class == owner
-
-          begin
-            # Otherwise, some inherited method could be preventing us
-            # from creating subclasses, so let's override it and rescue
-            owner.send(:define_method, :inherited) do |s|
-              inherited_method.call(s)
-            rescue
-              # Ignoring errors
-            end
-
-            # return a subclass
-            Class.new(constant)
-          ensure
-            # Reinstate the original inherited method back.
+          # Otherwise, some inherited method could be preventing us
+          # from creating subclasses, so let's override it and rescue
+          owner.send(:define_method, :inherited) do |new_subclass|
+            # Reinstate the original inherited method back ASAP
             owner.send(:define_method, :inherited, inherited_method)
+
+            # Register this new subclass ASAP, to prevent re-entry into the `create_safe_subclass` code-path.
+            # This can happen if the sig of the original `.inherited` method references the generic type itself.
+            @generic_instances[name] ||= new_subclass
+
+            # Call the original `.inherited` method, but rescue any errors that might be raised,
+            # which would have otherwise prevented our subclass from being created.
+            inherited_method.call(new_subclass)
+          rescue
+            # Ignoring errors
           end
+
+          # return a subclass
+          Class.new(constant)
         end
 
         sig { params(constant: Module).returns(T::Array[TypeVariableModule]) }

spec/tapioca/runtime/generic_type_registry_spec.rb

@@ -83,12 +83,10 @@ class GenericTypeRegistrySpec < Minitest::Spec
             _ = HasNonRecursiveInheritedSig[Object]
           end
 
-          it "FIXME: breaks from infinite recursion if the sig on .inherited uses the generic type" do
+          it "works for classes whose .inherited sig reference the generic type itself" do
             # Our swizzled implementation of the `.inherited` method needs to be carefully implemented to not fall into
             # infinite recursion when the sig for the method references the class that it's defined on.
-            assert_raises(SystemStackError) do
-              HasRecursiveInheritedSig[Object]
-            end
+            _ = HasRecursiveInheritedSig[Object]
           end
         end
       end
@@ -120,8 +118,6 @@ class HasNonRecursiveInheritedSig
         class << self
           extend T::Sig
 
-          # The correct type would be `T::Class[SampleGenericClass[T.anything]]`, but that would crash Tapioca.
-          # That's not honey Pooh, that's recursion!
           sig { params(subclass: T::Class[T.anything]).void }
           def inherited(subclass)
             super