diff --git a/.github/workflows/e2e-tests.yml b/.github/workflows/e2e-tests.yml
index b0f15a8a69d..2d48c37e3c0 100644
--- a/.github/workflows/e2e-tests.yml
+++ b/.github/workflows/e2e-tests.yml
@@ -254,6 +254,14 @@ jobs:
               cd e2e/bug-15102c
               composer install
               ../../bin/phpstan analyze
+          - script: |
+              cd e2e/bug-15184
+              composer install
+              # The crash needs OPcache: forked workers switch it off, the --debug main process and
+              # spawned workers (pcntl_fork disabled, as on Windows) keep it on.
+              php -r 'if (!extension_loaded("Zend OPcache")) { fwrite(STDERR, "OPcache is not loaded, so this step cannot reproduce the crash.\n"); exit(1); }'
+              ../../bin/phpstan analyze --debug
+              php -d disable_functions=pcntl_fork ../../bin/phpstan analyze
           - script: |
               cd e2e/bug-14724
               composer install
diff --git a/compiler/build/scoper.inc.php b/compiler/build/scoper.inc.php
index 6407e29f8c4..ac20519cee6 100644
--- a/compiler/build/scoper.inc.php
+++ b/compiler/build/scoper.inc.php
@@ -143,6 +143,7 @@ function (string $filePath, string $prefix, string $content): string {
 		function (string $filePath, string $prefix, string $content): string {
 			if (!in_array($filePath, [
 				'bin/phpstan',
+				'src/Reflection/BetterReflection/SourceLocator/AutoloadSourceLocator.php',
 				'src/Testing/TestCaseSourceLocatorFactory.php',
 				'src/Testing/PHPStanTestCase.php',
 				'vendor/ondrejmirtes/better-reflection/src/SourceLocator/Type/ComposerSourceLocator.php',
diff --git a/e2e/bug-15184/.gitignore b/e2e/bug-15184/.gitignore
new file mode 100644
index 00000000000..3a9875b460f
--- /dev/null
+++ b/e2e/bug-15184/.gitignore
@@ -0,0 +1,2 @@
+/vendor/
+composer.lock
diff --git a/e2e/bug-15184/composer.json b/e2e/bug-15184/composer.json
new file mode 100644
index 00000000000..d0f3d580af2
--- /dev/null
+++ b/e2e/bug-15184/composer.json
@@ -0,0 +1,10 @@
+{
+	"autoload": {
+		"psr-4": {
+			"Bug15184\\": "src/"
+		},
+		"files": [
+			"src/Type/optional.php"
+		]
+	}
+}
diff --git a/e2e/bug-15184/phpstan.dist.neon b/e2e/bug-15184/phpstan.dist.neon
new file mode 100644
index 00000000000..1290c812dc1
--- /dev/null
+++ b/e2e/bug-15184/phpstan.dist.neon
@@ -0,0 +1,11 @@
+parameters:
+	level: 8
+	paths:
+		- test.php
+	ignoreErrors:
+		# The analysis must report the unknown class, not die including src/Type/optional.php again.
+		-
+			message: '#^Class Bug15184\\Type\\optional not found\.$#'
+			identifier: class.notFound
+			count: 1
+			path: test.php
diff --git a/e2e/bug-15184/src/Type/optional.php b/e2e/bug-15184/src/Type/optional.php
new file mode 100644
index 00000000000..37c0ddad90e
--- /dev/null
+++ b/e2e/bug-15184/src/Type/optional.php
@@ -0,0 +1,9 @@
+<?php declare(strict_types = 1);
+
+namespace Bug15184\Type;
+
+// One function per file, loaded through Composer's "files" autoload, in a directory
+// the PSR-4 prefix also maps - the layout of php-standard-library and azjezz/psl.
+function optional(): void
+{
+}
diff --git a/e2e/bug-15184/test.php b/e2e/bug-15184/test.php
new file mode 100644
index 00000000000..107f58b1dd4
--- /dev/null
+++ b/e2e/bug-15184/test.php
@@ -0,0 +1,8 @@
+<?php declare(strict_types = 1);
+
+namespace Bug15184;
+
+function check(object $o): bool
+{
+	return $o instanceof \Bug15184\Type\optional;
+}
diff --git a/src/Reflection/BetterReflection/SourceLocator/AutoloadSourceLocator.php b/src/Reflection/BetterReflection/SourceLocator/AutoloadSourceLocator.php
index 0a5a61c3793..2509e54dd97 100644
--- a/src/Reflection/BetterReflection/SourceLocator/AutoloadSourceLocator.php
+++ b/src/Reflection/BetterReflection/SourceLocator/AutoloadSourceLocator.php
@@ -2,6 +2,7 @@
 
 namespace PHPStan\Reflection\BetterReflection\SourceLocator;
 
+use Composer\Autoload\ClassLoader;
 use Override;
 use ParseError;
 use PhpParser\Node\Arg;
@@ -31,12 +32,15 @@
 use function defined;
 use function function_exists;
 use function interface_exists;
+use function is_array;
 use function is_file;
 use function is_string;
 use function opcache_invalidate;
+use function realpath;
 use function restore_error_handler;
 use function set_error_handler;
 use function spl_autoload_functions;
+use function str_contains;
 use function strtolower;
 use function trait_exists;
 use const PHP_VERSION_ID;
@@ -336,14 +340,21 @@ private function locateClassByName(string $className): ?array
 		$this->silenceErrors();
 
 		try {
+			$foundByComposer = false;
 			$result = FileReadTrapStreamWrapper::withStreamWrapperOverride(
-				static function () use ($className): ?array {
+				static function () use ($className, &$foundByComposer): ?array {
 					$functions = spl_autoload_functions();
 					if ($functions === false) {
 						return null;
 					}
 
 					foreach ($functions as $preExistingAutoloader) {
+						$composerFile = self::findFileWithComposer($preExistingAutoloader, $className);
+						if ($composerFile !== null) {
+							$foundByComposer = true;
+							return [[$composerFile], $className, null];
+						}
+
 						try {
 							$preExistingAutoloader($className);
 						} catch (ParseError) {
@@ -370,7 +381,8 @@ static function () use ($className): ?array {
 				return null;
 			}
 
-			if (!function_exists('opcache_invalidate')) {
+			// nothing was included, so there is no trapped script to invalidate
+			if ($foundByComposer || !function_exists('opcache_invalidate')) {
 				return $result;
 			}
 
@@ -387,6 +399,39 @@ static function () use ($className): ?array {
 		}
 	}
 
+	/**
+	 * Asks Composer's ClassLoader which file loadClass() would include, without
+	 * including it. With OPcache holding a file this process already ran, an
+	 * include inside the trap runs the cached script instead of the trap's empty
+	 * one, and a file declaring a function dies with "Cannot redeclare".
+	 *
+	 * A path behind a stream wrapper is left to the trap: realpath() cannot
+	 * resolve it, and the phar's own ClassLoader maps PHPStan\ to phar://.
+	 *
+	 * @param callable(string): void $autoloader
+	 */
+	private static function findFileWithComposer(callable $autoloader, string $className): ?string
+	{
+		if (!is_array($autoloader) || !$autoloader[0] instanceof ClassLoader) {
+			return null;
+		}
+
+		$file = $autoloader[0]->findFile($className);
+		if ($file === false || str_contains($file, '://')) {
+			return null;
+		}
+
+		// findFile() can return ../ segments and mixed separators; resolve them
+		// as an include would, so that locateIdentifier() can compare the path
+		// with ReflectionClass::getFileName()
+		$resolvedFile = realpath($file);
+		if ($resolvedFile === false || !is_file($resolvedFile)) {
+			return null;
+		}
+
+		return $resolvedFile;
+	}
+
 	private function silenceErrors(): void
 	{
 		set_error_handler(static fn (): bool => true);