Cannot use null as default value for parameter in PHP-8

Question

In php-8 and older versions the following code works

class Foo {
    public function __construct(string $string = null) {}
}

But in php-8, along with property promotion, it throws an error

class Foo {
    public function __construct(private string $string = null) {}
}

Fatal error: Cannot use null as default value for parameter $string of type string

Making the string nullable works though

class Foo {
    public function __construct(private ?string $string = null) {}
}

So is this a bug too or intended behaviour?

Answer 1

See the RFC for Constructor Property Promotion

...because promoted parameters imply a property declaration, nullability must be explicitly declared, and is not inferred from a null default value:

class Test {
    // Error: Using null default on non-nullable property
    public function __construct(public Type $prop = null) {}
 
    // Correct: Make the type explicitly nullable instead
    public function __construct(public ?Type $prop = null) {}
}

Answer 2

This is not a bug!

class Foo {
    public function __construct(private string $string = null) {}
}

The above code is a short hand syntax to

class Foo {
    private string $string = null;
    public function __construct(string $string) {
        $this->string  = $string;
    }
}

Which generates a fatal error

Fatal error: Default value for property of type string may not be null.

So you can't initialize a typed property that is not nullable to NULL