Add non regression test

Author: VincentLanglet

tests/PHPStan/Rules/Operators/InvalidBinaryOperationRuleTest.php

@@ -795,6 +795,13 @@ public function testBenevolentUnion(): void
 		]);
 	}
 
+	public function testBug12123(): void
+	{
+		$this->checkImplicitMixed = true;
+
+		$this->analyse([__DIR__ . '/data/bug-12123.php'], []);
+	}
+
 	public function testBug7863(): void
 	{
 		$this->checkImplicitMixed = true;

tests/PHPStan/Rules/Operators/data/bug-12123.php

@@ -0,0 +1,7 @@
+<?php declare(strict_types = 1);
+
+namespace Bug12123;
+
+$x = gmp_init('1');
+$y = $x * 2;
+var_dump($y);