Fix phpstan/phpstan#8231: Missing type narrowing after (string)$value

src/Analyser/TypeSpecifier.php

@@ -2934,6 +2934,21 @@ private function resolveNormalizedIdentical(Expr\BinaryOp\Identical $expr, Scope
 			}
 		}
 
+		// (cast)$expr === value - propagate narrowing to inner expression
+		if (
+			!$context->null()
+			&& $unwrappedLeftExpr instanceof Expr\Cast
+		) {
+			$castProducingType = $this->determineCastProducingType($unwrappedLeftExpr, $rightType);
+			if ($castProducingType !== null) {
+				$innerExpr = $unwrappedLeftExpr->expr;
+				$result = $this->create($leftExpr, $rightType, $context, $scope)->setRootExpr($expr);
+				return $result->unionWith(
+					$this->create($innerExpr, $castProducingType, $context, $scope)->setRootExpr($expr),
+				);
+			}
+		}
+
 		$expressions = $this->findTypeExpressionsFromBinaryOperation($scope, $expr);
 		if ($expressions !== null) {
 			$exprNode = $expressions[0];
@@ -3123,4 +3138,40 @@ private function resolveNormalizedIdentical(Expr\BinaryOp\Identical $expr, Scope
 		return (new SpecifiedTypes([], []))->setRootExpr($expr);
 	}
 
+	/**
+	 * Given a cast expression and the value it's compared to,
+	 * returns the union of types that produce that value when cast.
+	 */
+	private function determineCastProducingType(Expr\Cast $cast, Type $comparedType): ?Type
+	{
+		if ($cast instanceof Expr\Cast\String_) {
+			$constantStrings = $comparedType->getConstantStrings();
+			if (count($constantStrings) === 1 && $constantStrings[0]->getValue() === '') {
+				// Types that produce '' when cast to string: null, false, ''
+				return TypeCombinator::union(
+					new NullType(),
+					new ConstantBooleanType(false),
+					new ConstantStringType(''),
+				);
+			}
+		}
+
+		if ($cast instanceof Expr\Cast\Int_) {
+			$constantScalars = $comparedType->getConstantScalarValues();
+			if (count($constantScalars) === 1 && $constantScalars[0] === 0) {
+				// Types that produce 0 when cast to int: null, false, 0, 0.0, '', '0'
+				return TypeCombinator::union(
+					new NullType(),
+					new ConstantBooleanType(false),
+					new ConstantIntegerType(0),
+					new ConstantFloatType(0.0),
+					new ConstantStringType(''),
+					new ConstantStringType('0'),
+				);
+			}
+		}
+
+		return null;
+	}
+
 }

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

@@ -0,0 +1,75 @@
+<?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);
+	}
+}
+
+// (int) cast narrowing
+function testIntCast(int|null $x): void {
+	if ((int)$x !== 0) {
+		assertType('int<min, -1>|int<1, max>', $x);
+	}
+}
+
+function testIntCastIdentical(int|null $x): void {
+	if ((int)$x === 0) {
+		assertType('0|null', $x);
+	} else {
+		assertType('int<min, -1>|int<1, max>', $x);
+	}
+}
+
+function testIntCastWithString(int|string|null $x): void {
+	if ((int)$x !== 0) {
+		assertType("int<min, -1>|int<1, max>|non-falsy-string", $x);
+	}
+}
+
+function testIntCastWithFloat(float|null $x): void {
+	if ((int)$x !== 0) {
+		assertType('float', $x);
+	}
+}
+
+function testIntCastWithBool(bool|int|null $x): void {
+	if ((int)$x !== 0) {
+		assertType('int<min, -1>|int<1, max>|true', $x);
+	}
+}