Fix constant-string handling in union-types

src/Type/UnionType.php

@@ -17,6 +17,7 @@
 use PHPStan\ShouldNotHappenException;
 use PHPStan\TrinaryLogic;
 use PHPStan\Type\Constant\ConstantBooleanType;
+use PHPStan\Type\Constant\ConstantStringType;
 use PHPStan\Type\Generic\GenericClassStringType;
 use PHPStan\Type\Generic\TemplateMixedType;
 use PHPStan\Type\Generic\TemplateType;
@@ -77,6 +78,24 @@ public function getTypes(): array
 		return $this->types;
 	}
 
+	/**
+	 * @param class-string<Type> $typeClass
+	 * @return list<Type>
+	 */
+	private function getTypesOfClass(string $typeClass): array
+	{
+		$matchingTypes = [];
+		foreach ($this->getTypes() as $innerType) {
+			if (!$innerType instanceof $typeClass) {
+				return [];
+			}
+
+			$matchingTypes[] = $innerType;
+		}
+
+		return $matchingTypes;
+	}
+
 	public function isNormalized(): bool
 	{
 		return $this->normalized;
@@ -117,7 +136,7 @@ public function getConstantArrays(): array
 
 	public function getConstantStrings(): array
 	{
-		return UnionTypeHelper::getConstantStrings($this->getTypes());
+		return UnionTypeHelper::getConstantStrings($this->getTypesOfClass(ConstantStringType::class));
 	}
 
 	public function accepts(Type $type, bool $strictTypes): TrinaryLogic

tests/PHPStan/Analyser/NodeScopeResolverTest.php

@@ -1154,6 +1154,7 @@ public function dataFileAsserts(): iterable
 			yield from $this->gatherAssertTypes(__DIR__ . '/data/pathinfo-php8.php');
 		}
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/pathinfo.php');
+		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-8568.php');
 	}
 
 	/**

tests/PHPStan/Analyser/data/bug-8568.php

@@ -0,0 +1,18 @@
+<?php declare(strict_types = 1);
+
+namespace Bug8568;
+
+use function PHPStan\Testing\assertType;
+
+class HelloWorld
+{
+	public function sayHello(): void
+	{
+		assertType('non-falsy-string', 'a' . $this->get());
+	}
+
+	public function get(): ?int
+	{
+		return rand() ? 5 : null;
+	}
+}