WIP

lib/graphql/schema/argument.rb

@@ -301,18 +301,28 @@ def coerce_into_values(parent_object, values, context, argument_values)
 
       private
 
-      def validate_input_type(input_type)
+      def validate_input_type(input_type, wrapped: false)
         if input_type.is_a?(String) || input_type.is_a?(GraphQL::Schema::LateBoundType)
           # Do nothing; assume this will be validated later
+          false
         elsif input_type.kind.non_null? || input_type.kind.list?
-          validate_input_type(input_type.unwrap)
+          type_ready = validate_input_type(input_type.unwrap, wrapped: true)
+          validate_default_value(default_value, input_type) if default_value? && type_ready && !wrapped
+          type_ready
         elsif !input_type.kind.input?
           raise ArgumentError, "Invalid input type for #{path}: #{input_type.graphql_name}. Must be scalar, enum, or input object, not #{input_type.kind.name}."
         else
-          # It's an input type, we're OK
+          # It's an input type, the type itself is OK but make sure the default conforms
+          validate_default_value(default_value, input_type) if default_value? && !wrapped
+          true
         end
       end
 
+      def validate_default_value(default_value, input_type)
+        default_value = input_type.coerce_isolated_result(default_value) unless default_value.nil?
+        raise "BOOM" unless input_type.valid_isolated_input?(default_value)
+      end
+
       def validate_deprecated_or_optional(null:, deprecation_reason:)
         if deprecation_reason && !null
           raise ArgumentError, "Required arguments cannot be deprecated: #{path}."

spec/graphql/schema/argument_spec.rb

@@ -209,6 +209,13 @@ def self.object_from_id(id, ctx)
       res = SchemaArgumentTest::Schema.execute(query_str)
       assert_equal "Argument 'requiredWithDefaultArg' on Field 'field' has an invalid value (null). Expected type 'Int!'.", res['errors'][0]['message']
     end
+
+    it 'validates the type of the default value' do
+      arg = GraphQL::Schema::Argument.new("my_arg", GraphQL::Types::Int, required: true, owner: nil, default_value: "a_string")
+      assert_raises(StandardError) do
+        arg.type
+      end
+    end
   end
 
   describe 'loads' do