Validate that deprecated argument are optional

lib/graphql/schema/validation.rb

@@ -133,6 +133,12 @@ def self.assert_named_items_are_valid(item_name, get_items_proc)
           end
         }
 
+        DEPRECATED_ARGUMENTS_ARE_OPTIONAL = ->(argument) {
+          if argument.deprecation_reason && argument.type.non_null?
+            "must be optional because it's deprecated"
+          end
+        }
+
         TYPE_IS_VALID_INPUT_TYPE = ->(type) {
           outer_type = type.type
           inner_type = outer_type.respond_to?(:unwrap) ? outer_type.unwrap : nil
@@ -265,8 +271,10 @@ def self.assert_named_items_are_valid(item_name, get_items_proc)
           Rules::NAME_IS_STRING,
           Rules::RESERVED_NAME,
           Rules::DESCRIPTION_IS_STRING_OR_NIL,
+          Rules.assert_property(:deprecation_reason, String, NilClass),
           Rules::TYPE_IS_VALID_INPUT_TYPE,
           Rules::DEFAULT_VALUE_IS_VALID_FOR_TYPE,
+          Rules::DEPRECATED_ARGUMENTS_ARE_OPTIONAL,
         ],
         GraphQL::BaseType => [
           Rules::NAME_IS_STRING,

spec/graphql/schema/validation_spec.rb

@@ -344,6 +344,30 @@ def assert_validation_warns(object, warning)
       end
     }
 
+    let(:deprecated_optional_argument) {
+      GraphQL::Argument.define do
+        name "Something"
+        deprecation_reason "Don't use me"
+        type GraphQL::INT_TYPE
+      end
+    }
+
+    let(:deprecated_required_argument) {
+      GraphQL::Argument.define do
+        name "Something"
+        deprecation_reason "Don't use me"
+        type !GraphQL::INT_TYPE
+      end
+    }
+
+    let(:invalid_deprecation_reason_argument) {
+      GraphQL::Argument.define do
+        name "Something"
+        deprecation_reason 1
+        type GraphQL::INT_TYPE
+      end
+    }
+
     it "requires the type is a Base type" do
       assert_error_includes untyped_argument, "must be a valid input type (Scalar or InputObject), not Symbol"
     end
@@ -359,6 +383,18 @@ def assert_validation_warns(object, warning)
     it "allows null default value for nullable argument" do
       assert_nil GraphQL::Schema::Validation.validate(null_default_value)
     end
+
+    it "allows deprecated optional arguments" do
+      assert_nil GraphQL::Schema::Validation.validate(deprecated_optional_argument)
+    end
+
+    it "disallows deprecated required arguments" do
+      assert_error_includes deprecated_required_argument, "must be optional because it's deprecated"
+    end
+
+    it "disallows non-string deprecation reasons" do
+      assert_error_includes invalid_deprecation_reason_argument, "deprecation_reason must return String or NilClass, not Integer (1)"
+    end
   end
 
   describe "validating instrumentation" do