Handle always-null LHS and always-terminating LHS in null coalescing assignment

When the LHS of ??= is always null, the RHS always executes, so the
expression should be considered always-terminating if the RHS is.
When the LHS evaluation itself always terminates (e.g. calling a
never-returning function), the whole expression is always-terminating.

Added tests for both cases as requested in review.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: phpstan-bot

src/Analyser/ExprHandler/AssignOpHandler.php

@@ -66,10 +66,11 @@ static function (MutatingScope $scope) use ($stmt, $expr, $nodeCallback, $contex
 				$exprResult = $nodeScopeResolver->processExprNode($stmt, $expr->expr, $scope, $storage, $nodeCallback, $context->enterDeep());
 				if ($expr instanceof Expr\AssignOp\Coalesce) {
 					$nodeScopeResolver->storeBeforeScope($storage, $expr, $originalScope);
+					$isAlwaysTerminating = $exprResult->isAlwaysTerminating() && $originalScope->getType($expr->var)->isNull()->yes();
 					return new ExpressionResult(
 						$exprResult->getScope()->mergeWith($originalScope),
 						$exprResult->hasYield(),
-						false,
+						$isAlwaysTerminating,
 						$exprResult->getThrowPoints(),
 						$exprResult->getImpurePoints(),
 					);

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

@@ -13,3 +13,11 @@ function test(string|null $test): void
 
 	assertType('string', $test);
 }
+
+function testAlwaysNull(): void
+{
+	$test = null;
+	$test ??= throw new Exception();
+
+	assertType('*NEVER*', $test);
+}

tests/PHPStan/Rules/DeadCode/UnreachableStatementRuleTest.php

@@ -400,7 +400,16 @@ public function testBug14328(): void
 	public function testBug14369(): void
 	{
 		$this->treatPhpDocTypesAsCertain = true;
-		$this->analyse([__DIR__ . '/data/bug-14369.php'], []);
+		$this->analyse([__DIR__ . '/data/bug-14369.php'], [
+			[
+				'Unreachable statement - code above always terminates.',
+				21,
+			],
+			[
+				'Unreachable statement - code above always terminates.',
+				28,
+			],
+		]);
 	}
 
 }

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

@@ -12,3 +12,24 @@ function test(string|null $test): void
 
 	echo $test;
 }
+
+function testAlwaysNull(): void
+{
+	$test = null;
+	$test ??= throw new Exception();
+
+	echo $test;
+}
+
+function testAlwaysTerminatingLhs(): void
+{
+	alwaysThrows()->prop ??= throw new Exception();
+
+	echo 'unreachable';
+}
+
+/** @return never */
+function alwaysThrows(): never
+{
+	throw new Exception();
+}