PHP8.2 Disjunctive Normal Form Types with a default value of null

[HypeMC]

Description

There seems to be some inconsistent behavior with Disjunctive Normal Form Types when an argument has a default value of null.

Usually if the default value is null, the argument is implicitly nullable, eg:

<?php
function foo(string $param = null) { var_dump($param); }
function bar(\Foo\Bar|\Foo\Baz $param = null) { var_dump($param); }

foo(null);
bar(null);

result:

NULL
NULL

https://3v4l.org/iBlne

An exception to this rule are PHP 8.1 intersection types:

function foo(\Foo\Bar&\Foo\Baz $param = null) { var_dump($param); }

foo(null);

result:

Fatal error: Cannot use null as default value for parameter $param of type Foo\Bar&Foo\Baz in /in/k0OjD on line 3

Process exited with code 255.

https://3v4l.org/k0OjD

However, the same error is raised on PHP 8.2, even though the following works:

function foo((\Foo\Bar&\Foo\Baz)|\Foo\Qux $param = null) { var_dump($param); }

foo(null);

result:

NULL

https://3v4l.org/UADDP

With the introduction of DNF types, I would expect \Foo\Bar&\Foo\Baz $param = null to implicitly mean (\Foo\Bar&\Foo\Baz)|null $param = null, which works as expected:

function foo((\Foo\Bar&\Foo\Baz)|null $param = null) { var_dump($param); }

foo(null);

result:

NULL

https://3v4l.org/gjOAp

PHP Version

PHP 8.2rc3

Operating System

Ubuntu 22.04