WIP

lib/graphql/schema/argument.rb

@@ -247,6 +247,9 @@ def coerce_into_values(parent_object, values, context, argument_values)
         elsif values.key?(arg_key)
           value = values[arg_key]
         elsif default_value?
+          context.schema.error_handler.with_error_handling(context) do
+            validate_default_value
+          end
           value = default_value
           default_used = true
         else
@@ -297,6 +300,16 @@ def coerce_into_values(parent_object, values, context, argument_values)
 
       private
 
+      def validate_default_value
+        return if @default_validated
+
+        coerced_value = default_value.nil? ? nil : type.coerce_isolated_result(default_value)
+        validation = type.validate_isolated_input(coerced_value)
+        raise GraphQL::RuntimeTypeError, validation.problems.first["explanation"] unless validation.valid?
+
+        @default_validated = true
+      end
+
       def validate_input_type(input_type)
         if input_type.is_a?(String) || input_type.is_a?(GraphQL::Schema::LateBoundType)
           # Do nothing; assume this will be validated later

lib/graphql/schema/member/validates_input.rb

@@ -20,6 +20,10 @@ def valid_isolated_input?(v)
           valid_input?(v, GraphQL::Query::NullContext)
         end
 
+        def validate_isolated_input(v)
+          validate_input(v, GraphQL::Query::NullContext)
+        end
+
         def coerce_isolated_input(v)
           coerce_input(v, GraphQL::Query::NullContext)
         end