Add possible_types check for unions that are used only for loads:

lib/graphql/schema/member/has_arguments.rb

@@ -359,7 +359,8 @@ def authorize_application_object(argument, id, context, loaded_application_objec
               if application_object.nil?
                 nil
               else
-                maybe_lazy_resolve_type = context.schema.resolve_type(argument.loads, application_object, context)
+                arg_loads_type = argument.loads
+                maybe_lazy_resolve_type = context.schema.resolve_type(arg_loads_type, application_object, context)
                 context.query.after_lazy(maybe_lazy_resolve_type) do |resolve_type_result|
                   if resolve_type_result.is_a?(Array) && resolve_type_result.size == 2
                     application_object_type, application_object = resolve_type_result
@@ -368,10 +369,17 @@ def authorize_application_object(argument, id, context, loaded_application_objec
                     # application_object is already assigned
                   end
 
-                  if !(
-                      context.types.possible_types(argument.loads).include?(application_object_type) ||
-                      context.types.loadable?(argument.loads, context)
-                    )
+                  passes_possible_types_check = if context.types.loadable?(arg_loads_type, context)
+                    if arg_loads_type.kind.union?
+                      # This union is used in `loads:` but not otherwise visible to this query
+                      context.types.loadable_possible_types(arg_loads_type, context).include?(application_object_type)
+                    else
+                      true
+                    end
+                  else
+                    context.types.possible_types(arg_loads_type).include?(application_object_type)
+                  end
+                  if !passes_possible_types_check
                     err = GraphQL::LoadApplicationObjectFailedError.new(context: context, argument: argument, id: id, object: application_object)
                     application_object = load_application_object_failed(err)
                   end

lib/graphql/schema/visibility/migration.rb

@@ -112,6 +112,7 @@ def loaded_types
           :all_types_h,
           :fields,
           :loadable?,
+          :loadable_possible_types,
           :type,
           :arguments,
           :argument,

lib/graphql/schema/visibility/profile.rb

@@ -84,6 +84,8 @@ def initialize(name: nil, context:, schema:)
           @cached_arguments = Hash.new do |h, owner|
             h[owner] = non_duplicate_items(owner.all_argument_definitions, @cached_visible_arguments)
           end.compare_by_identity
+
+          @loadable_possible_types = Hash.new { |h, union_type| h[union_type] = union_type.possible_types }.compare_by_identity
         end
 
         def field_on_visible_interface?(field, owner)
@@ -249,6 +251,10 @@ def loadable?(t, _ctx)
           !@all_types[t.graphql_name] && @cached_visible[t]
         end
 
+        def loadable_possible_types(t, _ctx)
+          @loadable_possible_types[t]
+        end
+
         def loaded_types
           @all_types.values
         end

lib/graphql/schema/warden.rb

@@ -72,6 +72,7 @@ def visible_type_membership?(tm, ctx); tm.visible?(ctx); end
           def interface_type_memberships(obj_t, ctx); obj_t.interface_type_memberships; end
           def arguments(owner, ctx); owner.arguments(ctx); end
           def loadable?(type, ctx); type.visible?(ctx); end
+          def loadable_possible_types(type, ctx); type.possible_types(ctx); end
           def visibility_profile
             @visibility_profile ||= Warden::VisibilityProfile.new(self)
           end
@@ -106,6 +107,7 @@ def fields(type_defn); type_defn.all_field_definitions; end # rubocop:disable De
         def get_field(parent_type, field_name); @schema.get_field(parent_type, field_name); end
         def reachable_type?(type_name); true; end
         def loadable?(type, _ctx); true; end
+        def loadable_possible_types(union_type, _ctx); union_type.possible_types; end
         def reachable_types; @schema.types.values; end # rubocop:disable Development/ContextIsPassedCop
         def possible_types(type_defn); @schema.possible_types(type_defn, Query::NullContext.instance, false); end
         def interfaces(obj_type); obj_type.interfaces; end
@@ -180,6 +182,10 @@ def loadable?(t, ctx) # TODO remove ctx here?
           @warden.loadable?(t, ctx)
         end
 
+        def loadable_possible_types(t, ctx)
+          @warden.loadable_possible_types(t, ctx)
+        end
+
         def reachable_type?(type_name)
           !!@warden.reachable_type?(type_name)
         end
@@ -204,7 +210,7 @@ def initialize(context:, schema:)
           @visible_possible_types = @visible_fields = @visible_arguments = @visible_enum_arrays =
           @visible_enum_values = @visible_interfaces = @type_visibility = @type_memberships =
           @visible_and_reachable_type = @unions = @unfiltered_interfaces =
-          @reachable_type_set = @visibility_profile =
+          @reachable_type_set = @visibility_profile = @loadable_possible_types =
             nil
         @skip_warning = schema.plugins.any? { |(plugin, _opts)| plugin == GraphQL::Schema::Warden }
       end
@@ -229,6 +235,13 @@ def loadable?(type, _ctx)
         !reachable_type_set.include?(type) && visible_type?(type)
       end
 
+      def loadable_possible_types(union_type, _ctx)
+        @loadable_possible_types ||= read_through do |t|
+          t.possible_types # unfiltered
+        end
+        @loadable_possible_types[union_type]
+      end
+
       # @return [GraphQL::BaseType, nil] The type named `type_name`, if it exists (else `nil`)
       def get_type(type_name)
         @visible_types ||= read_through do |name|

spec/graphql/schema/union_spec.rb

@@ -456,7 +456,8 @@ def self.resolve_type(abs_type, obj, ctx)
       assert_equal "Video", res["data"]["mediaItemType"]
 
       res = UnionLoadsSchema.execute(query_str, variables: { mediaId: "Post/Year in Review" })
-      assert_nil res["data"]
+      assert_nil res["data"]["mediaItemType"]
+      assert_equal ["No object found for `id: \"Post/Year in Review\"`"], res["errors"].map { |e| e["message"] }
     end
   end
 end