Mixed, by-reference varargs parameter gets handled like an intersection type and throws a fatal error

[asmundstavdahl]

• PHP Depend version: 2.11.1
• PHP Version: 7.4, 8.0, 8.1
• Installation type: composer
• Operating System / Distribution & Version: Ubuntu 20.04.5

Current Behavior

Parsing a file that contains a function signature with a mixed-type by-reference varargs parameter (e.g. function f(mixed &...$vars){} throws the following error:

PHP Fatal error:  Uncaught TypeError: Argument 1 passed to PDepend\Source\AST\AbstractASTNode::addChild() must implement interface PDepend\Source\AST\ASTNode, null given, called in …/vendor/pdepend/pdepend/src/main/php/PDepend/Source/Language/PHP/PHPParserVersion81.php on line 198 and defined in …/vendor/pdepend/pdepend/src/main/php/PDepend/Source/AST/AbstractASTNode.php:413
Stack trace:
#0 …/vendor/pdepend/pdepend/src/main/php/PDepend/Source/Language/PHP/PHPParserVersion81.php(198): PDepend\Source\AST\AbstractASTNode->addChild()
#1 …/vendor/pdepend/pdepend/src/main/php/PDepend/Source/Language/PHP/PHPParserVersion81.php(225): PDepend\Source\Language\PHP\PHPParserVersion81->parseIntersectionTypeHint()
#2 …/vendor/pdepend/pdepend/src/main/php/PDepend/Source/Language/PHP/PHPParserVersion81.php(238): PDepend\Source\Language\PHP\PHPParserVersion81->parseIntersectionOrUnionTypeHint()
#3 /ho in …/vendor/pdepend/pdepend/src/main/php/PDepend/Source/AST/AbstractASTNode.php on line 413

Some debugging reveals that the parameter is parsed as an intersection of mixed and Tokens::T_ELLIPSIS, which does not make sense.

Expected Behavior

It is a valid function signature, so parsing should succeed.

Steps To Reproduce:

1. Make a file test.php with the contents <?php function f(mixed &...$vars){}
2. Run pdepend on test.php

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