Add unit test for #13048

Author: SplotyCode

tests/PHPStan/Rules/Comparison/MatchExpressionRuleTest.php

@@ -435,4 +435,11 @@ public function testPropertyHooks(): void
 		]);
 	}
 
+
+	#[RequiresPhp('>= 8.0')]
+	public function testBug13048(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-13048.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Comparison/data/bug-13048.php

@@ -0,0 +1,22 @@
+<?php // lint >= 8.0
+
+namespace Bug11852;
+
+enum IndexBy {
+	case A;
+	case B;
+}
+
+/**
+ * @template T of IndexBy|null
+ * @param T $indexBy
+ * @return (T is null ? null : string)
+ */
+function run(?IndexBy $indexBy = null): ?string
+{
+	return match ($indexBy) {
+		IndexBy::A => 'by A',
+		IndexBy::B => 'by B',
+		null => null,
+	};
+}