Fix phpstan/phpstan#14369: Null coalescing assignment with throw not always terminating

- Fixed AssignOpHandler to not propagate isAlwaysTerminating from the RHS of ??=
- The RHS of ??= only executes when the LHS is null, so the expression cannot always terminate
- Added NSRT regression test verifying type narrowing after ??= throw
- Added UnreachableStatementRule test verifying no false positive unreachable code

Author: phpstan-bot

src/Analyser/ExprHandler/AssignOpHandler.php

@@ -69,7 +69,7 @@ static function (MutatingScope $scope) use ($stmt, $expr, $nodeCallback, $contex
 					return new ExpressionResult(
 						$exprResult->getScope()->mergeWith($originalScope),
 						$exprResult->hasYield(),
-						$exprResult->isAlwaysTerminating(),
+						false,
 						$exprResult->getThrowPoints(),
 						$exprResult->getImpurePoints(),
 					);

tests/PHPStan/Analyser/nsrt/bug-14369.php

@@ -0,0 +1,15 @@
+<?php // lint >= 8.0
+
+declare(strict_types = 1);
+
+namespace Bug14369;
+
+use Exception;
+use function PHPStan\Testing\assertType;
+
+function test(string|null $test): void
+{
+	$test ??= throw new Exception();
+
+	assertType('string', $test);
+}

tests/PHPStan/Rules/DeadCode/UnreachableStatementRuleTest.php

@@ -397,4 +397,10 @@ public function testBug14328(): void
 		]);
 	}
 
+	public function testBug14369(): void
+	{
+		$this->treatPhpDocTypesAsCertain = true;
+		$this->analyse([__DIR__ . '/data/bug-14369.php'], []);
+	}
+
 }

tests/PHPStan/Rules/DeadCode/data/bug-14369.php

@@ -0,0 +1,14 @@
+<?php // lint >= 8.0
+
+declare(strict_types = 1);
+
+namespace Bug14369DeadCode;
+
+use Exception;
+
+function test(string|null $test): void
+{
+	$test ??= throw new Exception();
+
+	echo $test;
+}