Fix missing type narrowing after (string)$value comparison

- Added handling in TypeSpecifier::resolveNormalizedIdentical for Cast\String_ expressions
- When (string)$x === '' or (string)$x !== '', propagate narrowing to inner expression $x
- Types that produce '' when cast to string (null, false, '') are used as the narrowing type
- New regression test in tests/PHPStan/Analyser/nsrt/bug-8231.php

Author: github-actions[bot]

src/Analyser/TypeSpecifier.php

@@ -2934,6 +2934,27 @@ private function resolveNormalizedIdentical(Expr\BinaryOp\Identical $expr, Scope
 			}
 		}
 
+		// (string)$expr === '' - propagate narrowing to inner expression
+		if (
+			!$context->null()
+			&& $unwrappedLeftExpr instanceof Expr\Cast\String_
+		) {
+			$rightConstantStrings = $rightType->getConstantStrings();
+			if (count($rightConstantStrings) === 1 && $rightConstantStrings[0]->getValue() === '') {
+				// Types that produce '' when cast to string: null, false, ''
+				$castToEmptyStringType = TypeCombinator::union(
+					new NullType(),
+					new ConstantBooleanType(false),
+					new ConstantStringType(''),
+				);
+				$innerExpr = $unwrappedLeftExpr->expr;
+				$result = $this->create($leftExpr, $rightType, $context, $scope)->setRootExpr($expr);
+				return $result->unionWith(
+					$this->create($innerExpr, $castToEmptyStringType, $context, $scope)->setRootExpr($expr),
+				);
+			}
+		}
+
 		$expressions = $this->findTypeExpressionsFromBinaryOperation($scope, $expr);
 		if ($expressions !== null) {
 			$exprNode = $expressions[0];

tests/PHPStan/Analyser/nsrt/bug-8231.php

@@ -0,0 +1,42 @@
+<?php // lint >= 8.0
+
+declare(strict_types = 1);
+
+namespace Bug8231;
+
+use function PHPStan\Testing\assertType;
+
+function foo(string $x): void {}
+
+function test(string|null $x): void {
+	if ((string)$x !== '') {
+		assertType('non-empty-string', $x);
+		foo($x);
+	}
+}
+
+function testIdentical(string|null $x): void {
+	if ((string)$x === '') {
+		assertType("''|null", $x);
+	} else {
+		assertType('non-empty-string', $x);
+	}
+}
+
+function testInt(int|null $x): void {
+	if ((string)$x !== '') {
+		assertType('int', $x);
+	}
+}
+
+function testIntString(int|string|null $x): void {
+	if ((string)$x !== '') {
+		assertType('int|non-empty-string', $x);
+	}
+}
+
+function testBool(bool|string|null $x): void {
+	if ((string)$x !== '') {
+		assertType('non-empty-string|true', $x);
+	}
+}