Allow deprecation of input values

As per the [September 2025 spec](https://github.com/graphql/graphql-spec/blob/main/changelogs/September2025.md).

Also makes `includeDeprecated` arguments non-nullable booleans, as well as the `reason` argument on the `@deprecated` directive.

Author: hugo-vrijswijk

modules/core/src/main/scala/compiler.scala

@@ -1146,7 +1146,12 @@ object QueryCompiler {
           Elab.transformChild(child => if (include) child else Filter(Eql(FieldTypeRef / "isDeprecated", Const(false)), child))
         case (TypeTypeRef, "enumValues", List(Binding("includeDeprecated", BooleanValue(include)))) =>
           Elab.transformChild(child => if (include) child else Filter(Eql(EnumValueTypeRef / "isDeprecated", Const(false)), child))
-
+        case (TypeTypeRef, "inputFields", List(Binding("includeDeprecated", BooleanValue(include)))) =>
+          Elab.transformChild(child => if (include) child else Filter(Eql(Introspection.__InputValueType / "isDeprecated", Const(false)), child))
+        case (FieldTypeRef, "args", List(Binding("includeDeprecated", BooleanValue(include)))) =>
+          Elab.transformChild(child => if (include) child else Filter(Eql(Introspection.__InputValueType / "isDeprecated", Const(false)), child))
+        case (Introspection.__DirectiveType, "args", List(Binding("includeDeprecated", BooleanValue(include)))) =>
+          Elab.transformChild(child => if (include) child else Filter(Eql(Introspection.__InputValueType / "isDeprecated", Const(false)), child))
         case _ =>
           Elab.unit
       }

modules/core/src/main/scala/introspection.scala

@@ -42,29 +42,29 @@ object Introspection {
           name: String
           description: String
 
-          # OBJECT and INTERFACE only
-          fields(includeDeprecated: Boolean = false): [__Field!]
+          # must be non-null for OBJECT and INTERFACE, otherwise null.
+          fields(includeDeprecated: Boolean! = false): [__Field!]
 
-          # OBJECT only
+          # must be non-null for OBJECT and INTERFACE, otherwise null.
           interfaces: [__Type!]
 
-          # INTERFACE and UNION only
+          # must be non-null for INTERFACE and UNION, otherwise null.
           possibleTypes: [__Type!]
 
-          # ENUM only
-          enumValues(includeDeprecated: Boolean = false): [__EnumValue!]
+          # must be non-null for ENUM, otherwise null.
+          enumValues(includeDeprecated: Boolean! = false): [__EnumValue!]
 
-          # INPUT_OBJECT only
-          inputFields: [__InputValue!]
+          # must be non-null for INPUT_OBJECT, otherwise null.
+          inputFields(includeDeprecated: Boolean! = false): [__InputValue!]
 
-          # NON_NULL and LIST only
+          # must be non-null for NON_NULL and LIST, otherwise null.
           ofType: __Type
         }
 
         type __Field {
           name: String!
           description: String
-          args: [__InputValue!]!
+          args(includeDeprecated: Boolean! = false): [__InputValue!]!
           type: __Type!
           isDeprecated: Boolean!
           deprecationReason: String
@@ -75,6 +75,8 @@ object Introspection {
           description: String
           type: __Type!
           defaultValue: String
+          isDeprecated: Boolean!
+          deprecationReason: String
         }
 
         type __EnumValue {
@@ -99,7 +101,7 @@ object Introspection {
           name: String!
           description: String
           locations: [__DirectiveLocation!]!
-          args: [__InputValue!]!
+          args(includeDeprecated: Boolean! = false): [__InputValue!]!
           isRepeatable: Boolean!
         }
 
@@ -111,6 +113,7 @@ object Introspection {
           FRAGMENT_DEFINITION
           FRAGMENT_SPREAD
           INLINE_FRAGMENT
+          VARIABLE_DEFINITION
           SCHEMA
           SCALAR
           OBJECT
@@ -258,7 +261,9 @@ object Introspection {
           ValueField("name", _.name),
           ValueField("description", _.description),
           ValueField("type", _.tpe.dealias),
-          ValueField("defaultValue", _.defaultValue.map(SchemaRenderer.renderValue))
+          ValueField("defaultValue", _.defaultValue.map(SchemaRenderer.renderValue)),
+          ValueField("isDeprecated", _.isDeprecated),
+          ValueField("deprecationReason", _.deprecationReason)
         ),
         ValueObjectMapping(__EnumValueType).on[EnumValueDefinition](
           ValueField("name", _.name),

modules/core/src/main/scala/schema.scala

@@ -764,6 +764,21 @@ sealed trait TypeWithFields extends NamedType {
   @deprecated("Use interfaces instead", "0.20.0")
   def allInterfaces: List[NamedType] = interfaces
 }
+/**
+ * Mixin for types that can be deprecated.
+ */
+protected trait Deprecatable {
+  def directives: List[Directive]
+
+  def deprecatedDirective: Option[Directive] =
+    directives.find(_.name == "deprecated")
+  def isDeprecated: Boolean = deprecatedDirective.isDefined
+  def deprecationReason: Option[String] =
+    for {
+      dir    <- deprecatedDirective
+      reason <- dir.args.collectFirst { case Binding("reason", StringValue(reason)) => reason }
+    } yield reason
+}
 
 /**
   * Scalar extensions allow additional directives to be applied to a pre-existing Scalar type
@@ -897,16 +912,7 @@ case class EnumValueDefinition(
   name: String,
   description: Option[String],
   directives: List[Directive]
-) {
-  def deprecatedDirective: Option[Directive] =
-    directives.find(_.name == "deprecated")
-  def isDeprecated: Boolean = deprecatedDirective.isDefined
-  def deprecationReason: Option[String] =
-    for {
-      dir    <- deprecatedDirective
-      reason <- dir.args.collectFirst { case Binding("reason", StringValue(reason)) => reason }
-    } yield reason
-}
+) extends Deprecatable
 
 /**
  * Input objects are composite types used as inputs into queries defined as a list of named input
@@ -972,16 +978,7 @@ case class Field(
   args: List[InputValue],
   tpe: Type,
   directives: List[Directive]
-) {
-  def deprecatedDirective: Option[Directive] =
-    directives.find(_.name == "deprecated")
-  def isDeprecated: Boolean = deprecatedDirective.isDefined
-  def deprecationReason: Option[String] =
-    for {
-      dir    <- deprecatedDirective
-      reason <- dir.args.collectFirst { case Binding("reason", StringValue(reason)) => reason }
-    } yield reason
-}
+) extends Deprecatable
 
 /**
  * @param defaultValue a String encoding (using the GraphQL language) of the default value used by
@@ -993,7 +990,7 @@ case class InputValue(
   tpe: Type,
   defaultValue: Option[Value],
   directives: List[Directive]
-)
+) extends Deprecatable
 
 sealed trait Value
 
@@ -1249,9 +1246,9 @@ object DirectiveDef {
            |fields on a type or deprecated enum values.
         """.stripMargin.trim
       ),
-      List(InputValue("reason", Some("Explains why this element was deprecated, usually also including a suggestion for how to access supported similar data. Formatted using the Markdown syntax, as specified by [CommonMark](https://commonmark.org/)."), NullableType(StringType), Some(StringValue("No longer supported")), Nil)),
+      List(InputValue("reason", Some("Explains why this element was deprecated, usually also including a suggestion for how to access supported similar data. Formatted using the Markdown syntax, as specified by [CommonMark](https://commonmark.org/)."), StringType, Some(StringValue("No longer supported")), Nil)),
       false,
-      List(DirectiveLocation.FIELD_DEFINITION, DirectiveLocation.ENUM_VALUE)
+      List(DirectiveLocation.FIELD_DEFINITION, DirectiveLocation.ARGUMENT_DEFINITION, DirectiveLocation.INPUT_FIELD_DEFINITION, DirectiveLocation.ENUM_VALUE)
     )
 
   val builtIns: List[DirectiveDef] =

modules/core/src/test/scala/compiler/DirectivesSuite.scala

@@ -149,6 +149,40 @@ final class DirectivesSuite extends CatsEffectSuite {
     assertEquals(schema.map(testDirectiveDefs), List(expected).success)
   }
 
+  test("Directive definition with deprecated argument") {
+    val expected =
+      DirectiveDef(
+        "foo",
+        None,
+        List(
+          InputValue(
+            "arg0",
+            None,
+            ScalarType.StringType,
+            None,
+            List(Directive("deprecated", List(Binding("reason", Value.StringValue("Use arg1 instead")))))
+          ),
+          InputValue("arg1", None, ScalarType.StringType, None, Nil)
+        ),
+        false,
+        List(FIELD)
+      )
+
+    val schema =
+      Schema("""
+        type Query {
+          foo: Int
+        }
+
+        directive @foo(
+          arg0: String! @deprecated(reason: "Use arg1 instead"),
+          arg1: String!
+        ) on FIELD
+      """)
+
+    assertEquals(schema.map(testDirectiveDefs), List(expected).success)
+  }
+
   test("Schema with directives") {
     val schema =
     """|schema @foo {

modules/core/src/test/scala/directives/DirectiveValidationSuite.scala

@@ -177,6 +177,7 @@ final class DirectiveValidationSuite extends CatsEffectSuite {
             f10: Int @deprecated
             f11: Int @deprecated(reason: 1)
             f12: Int @deprecated(x: "foo")
+            f13: Int @deprecated(reason: null)
           }
 
           directive @withArg(i: Int) on FIELD_DEFINITION
@@ -193,6 +194,7 @@ final class DirectiveValidationSuite extends CatsEffectSuite {
         Problem("""Unknown argument(s) 'x' in directive withRequiredArg"""),
         Problem("""Expected String found '1' for 'reason' in directive deprecated"""),
         Problem("""Unknown argument(s) 'x' in directive deprecated"""),
+        Problem("""Expected String found 'null' for 'reason' in directive deprecated"""),
       )
 
     assertEquals(schema.toProblems, problems)