Reject a computed field argument default its type cannot represent

docs/computed-fields.rst

@@ -193,7 +193,11 @@ GraphQL arguments are named, so the argument is ``year``, not a position.
 * A variadic parameter, or one passed by reference, cannot be an argument.
 
 These are checked when the metadata is built, and throw
-``ApiSkeletons\Doctrine\ORM\GraphQL\Exception\Metadata``.
+``ApiSkeletons\Doctrine\ORM\GraphQL\Exception\Metadata``.  When the type
+is built, an argument's type must be one which can be input, and a default
+must be a value of it: ``'2020-01-01'`` is not a value of ``date_immutable``,
+whose values are dates, so a parameter given that type in ``args`` may not
+have it as its default.  These throw the same exception.
 
 Each set of arguments has its own value, so aliases of a field with
 different arguments, such as ``a: totalRecordings(year: 2002)

src/Type/Entity/Entity.php

@@ -24,12 +24,14 @@
 use Doctrine\ORM\Mapping\ClassMetadata;
 use Doctrine\ORM\Mapping\MappingException;
 use GraphQL\Type\Definition\InputType;
+use GraphQL\Type\Definition\LeafType;
 use GraphQL\Type\Definition\NullableType;
 use GraphQL\Type\Definition\ObjectType;
 use GraphQL\Type\Definition\Type;
 use Laminas\Hydrator\HydratorInterface;
 use League\Event\EventDispatcher;
 use ReflectionClass;
+use Throwable;
 
 use function array_keys;
 use function array_merge;
@@ -44,6 +46,7 @@
 use function str_starts_with;
 use function substr;
 use function ucwords;
+use function var_export;
 
 /**
  * This class is used to build an ObjectType for an entity
@@ -492,12 +495,44 @@ private function computedFieldArgs(string $fieldName, ComputedFieldMetadata $com
                 continue;
             }
 
+            // A default is a value of the argument's type, which introspection
+            // serializes; the method receives it as a value of the type
+            if (! self::isValueOf($type, $argument->default)) {
+                throw new MetadataException(
+                    'Argument ' . $name . ' of computed field ' . $fieldName . ' of entity ' . $this->getEntityClass()
+                    . ' has the default value ' . var_export($argument->default, true) . ', which is not a value of its '
+                    . 'type ' . $type->toString() . '.  Remove the default, or give the parameter a type of which '
+                    . 'it is a value.',
+                );
+            }
+
             $args[$name]['defaultValue'] = $argument->default;
         }
 
         return ['args' => $args];
     }
 
+    /**
+     * Whether a scalar is a value of a type: the type is a scalar or an enum,
+     * not a list or an input object, and serializes the value
+     */
+    private static function isValueOf(Type $type, int|float|string|bool $value): bool
+    {
+        $type = Type::getNullableType($type);
+
+        if (! $type instanceof LeafType) {
+            return false;
+        }
+
+        try {
+            $type->serialize($value);
+        } catch (Throwable) {
+            return false;
+        }
+
+        return true;
+    }
+
     /**
      * The type of a computed field: its type, or a list of it
      */

test/Entity/ComputedArgsInvalid.php

@@ -20,6 +20,8 @@
 #[GraphQL\Entity(group: 'ComputedArgsObjectDefault')]
 #[GraphQL\Entity(group: 'ComputedArgsUnknownArg')]
 #[GraphQL\Entity(group: 'ComputedArgsNotInput')]
+#[GraphQL\Entity(group: 'ComputedArgsTypedDefault')]
+#[GraphQL\Entity(group: 'ComputedArgsListDefault')]
 #[ORM\Entity]
 class ComputedArgsInvalid
 {
@@ -75,4 +77,18 @@ public function getNotInput(mixed $page): int
     {
         return 0;
     }
+
+    /** A default which is not a value of the type its args give it */
+    #[GraphQL\ComputedField(type: 'string', group: 'ComputedArgsTypedDefault', args: ['since' => 'date_immutable'])]
+    public function getTypedDefault(mixed $since = '2020-01-01'): string
+    {
+        return (string) $since;
+    }
+
+    /** A scalar default for a list */
+    #[GraphQL\ComputedField(type: 'int', group: 'ComputedArgsListDefault', args: ['tags' => 'simple_array'])]
+    public function getListDefault(mixed $tags = 'live'): int
+    {
+        return 0;
+    }
 }

test/Feature/ComputedField/ComputedFieldArgumentsTest.php

@@ -357,6 +357,17 @@ public static function invalidProvider(): array
                 'The args of the ComputedField attribute of computed field method getUnknownArg of entity '
                 . ComputedArgsInvalid::class . ' give the type of other, which is not a parameter of the method.',
             ],
+            'default not a value of the given type' => [
+                'ComputedArgsTypedDefault',
+                'Argument since of computed field typedDefault of entity ' . ComputedArgsInvalid::class
+                . " has the default value '2020-01-01', which is not a value of its type DateImmutable.  Remove "
+                . 'the default, or give the parameter a type of which it is a value.',
+            ],
+            'scalar default for a list' => [
+                'ComputedArgsListDefault',
+                'Argument tags of computed field listDefault of entity ' . ComputedArgsInvalid::class
+                . " has the default value 'live', which is not a value of its type [String].",
+            ],
             'not an input type' => [
                 'ComputedArgsNotInput',
                 'Argument page of computed field notInput of entity ' . ComputedArgsInvalid::class