Add test to show the problem

Author: kubawerlos

tests/end-to-end/regression/5891.phpt

@@ -0,0 +1,20 @@
+--TEST--
+https://github.com/sebastianbergmann/phpunit/issues/5891
+--FILE--
+<?php declare(strict_types=1);
+$_SERVER['argv'][] = '--do-not-cache-result';
+$_SERVER['argv'][] = '--no-configuration';
+$_SERVER['argv'][] = __DIR__ . '/5891/Issue5891Test.php';
+
+require_once __DIR__ . '/../../bootstrap.php';
+(new PHPUnit\TextUI\Application)->run($_SERVER['argv']);
+--EXPECTF--
+PHPUnit %s by Sebastian Bergmann and contributors.
+
+Runtime: %s
+
+..                                                                  2 / 2 (100%)
+
+Time: %s, Memory: %s MB
+
+OK (2 tests, 3 assertions)

tests/end-to-end/regression/5891/Issue5891Test.php

@@ -0,0 +1,55 @@
+<?php declare(strict_types=1);
+/*
+ * This file is part of PHPUnit.
+ *
+ * (c) Sebastian Bergmann <[email protected]>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+namespace PHPUnit\TestFixture\Issue5891;
+
+use PHPUnit\Framework\TestCase;
+
+final class Issue5891Test extends TestCase
+{
+    public function testVariadicParam(): void
+    {
+        $variadicParam = $this->createMock(VariadicParam::class);
+        $variadicParam
+            ->method('foo')
+            ->with($this->callback(static function (...$items): bool
+            {
+                self::assertSame([1], $items); // should be "self::assertSame([1, 2, 3], $items);"
+
+                return true;
+            }));
+
+        $variadicParam->foo(1, 2, 3);
+    }
+
+    public function testTwoParametersAndVariadicParam(): void
+    {
+        $twoParametersAndVariadicParam = $this->createMock(TwoParametersAndVariadicParam::class);
+        $twoParametersAndVariadicParam
+            ->method('foo')
+            ->with($this->callback(static function ($head, ...$tail): bool
+            {
+                self::assertSame('1st', $head);
+                self::assertSame([], $tail); // should be "self::assertSame(['2nd', '3rd', '4th'], $tail);"
+
+                return true;
+            }));
+
+        $twoParametersAndVariadicParam->foo('1st', '2nd', '3rd', '4th');
+    }
+}
+
+interface VariadicParam
+{
+    public function foo(int ...$items): void;
+}
+interface TwoParametersAndVariadicParam
+{
+    public function foo(string $first, string $second, string ...$rest): void;
+}