NarrowReturnType: preg_* narrowing only for constant patterns

Author: dg

extension-narrowReturnType.neon

@@ -4,7 +4,7 @@ services:
 			# PHP still returns |false, but it's trivial — indicates a programming error or unrealistic state
 			# ---------
 
-			# Regex (false = invalid pattern)
+			# Regex (false = invalid pattern; only stripped when pattern is a constant string)
 			preg_match
 			preg_split
 			preg_grep

src/Type/NarrowReturnTypeResolver.php

@@ -17,7 +17,7 @@
 use PHPStan\Type\ExpressionTypeResolverExtension;
 use PHPStan\Type\Type;
 use PHPStan\Type\TypeCombinator;
-use function array_merge, explode, str_contains, strtolower;
+use function array_merge, explode, str_contains, str_starts_with, strtolower;
 
 
 /**
@@ -81,10 +81,20 @@ private function resolveFuncCall(FuncCall $expr, Scope $scope): ?Type
 		}
 
 		$functionReflection = $this->reflectionProvider->getFunction($expr->name, $scope);
-		if (!isset($this->functions[$functionReflection->getName()])) {
+		$functionName = $functionReflection->getName();
+
+		if (!isset($this->functions[$functionName])) {
 			return null;
 		}
 
+		// preg_* functions return false only for invalid patterns, so skip narrowing for non-constant patterns
+		if (str_starts_with($functionName, 'preg_')) {
+			$args = $expr->getArgs();
+			if ($args === [] || $scope->getType($args[0]->value)->getConstantStrings() === []) {
+				return null;
+			}
+		}
+
 		$parametersAcceptor = ParametersAcceptorSelector::selectFromArgs(
 			$scope,
 			$expr->getArgs(),

tests/data/narrow-return-type.php

@@ -27,15 +27,24 @@
 // JSON
 assertType('non-empty-string', json_encode('data'));
 
-// Regex
-function testRegex(string $s): void
+// Regex (constant pattern — |false stripped)
+function testRegexConstant(string $s): void
 {
 	assertType('0|1', preg_match('/a/', $s));
 	assertType('list<string>', preg_split('/a/', $s));
 	assertType('array', preg_grep('/a/', [$s]));
 }
 
 
+// Regex (non-constant pattern — |false preserved)
+function testRegexDynamic(string $pattern, string $s): void
+{
+	assertType('0|1|false', preg_match($pattern, $s));
+	assertType('list<string>|false', preg_split($pattern, $s));
+	assertType('array|false', preg_grep($pattern, [$s]));
+}
+
+
 // mb_string
 assertType('string', mb_chr(65));
 assertType('int', mb_ord('A'));