Add non regression test for #8620

tests/PHPStan/Analyser/NodeScopeResolverTest.php

@@ -1155,6 +1155,7 @@ public function dataFileAsserts(): iterable
 		}
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/pathinfo.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-8568.php');
+		yield from $this->gatherAssertTypes(__DIR__ . '/../Rules/DeadCode/data/bug-8620.php');
 	}
 
 	/**

tests/PHPStan/Rules/DeadCode/UnreachableStatementRuleTest.php

@@ -131,4 +131,10 @@ public function testBug7188(): void
 		]);
 	}
 
+	public function testBug8620(): void
+	{
+		$this->treatPhpDocTypesAsCertain = true;
+		$this->analyse([__DIR__ . '/data/bug-8620.php'], []);
+	}
+
 }

tests/PHPStan/Rules/DeadCode/data/bug-8620.php

@@ -0,0 +1,33 @@
+<?php declare(strict_types = 1);
+
+namespace Bug8620;
+
+use function PHPStan\Testing\assertType;
+
+class HelloWorld
+{
+	public function nullCoalesceAndConcatenation (?int $a = null): int
+	{
+		$key = ($a ?? "x") . "-";
+		assertType('non-falsy-string', $key);
+		if ($key === "x-") { return 0; }
+
+		return 1;
+	}
+
+	public function nullCoalesce (?int $a = null): int
+	{
+		$key = ($a ?? "");
+		assertType("''|int", $key);
+		if ($key === "") { return 0; }
+
+		return 1;
+	}
+
+	public function nullCheck (?int $a = null): int
+	{
+		if (is_null ($a)) { return 0; }
+
+		return 1;
+	}
+}