diff --git a/.github/workflows/e2e-tests.yml b/.github/workflows/e2e-tests.yml
index 188ecf899e..1ab8c24c1e 100644
--- a/.github/workflows/e2e-tests.yml
+++ b/.github/workflows/e2e-tests.yml
@@ -143,6 +143,10 @@ jobs:
           - script: |
               cd e2e/bug-15102b
               ../../bin/phpstan analyze
+          - script: |
+              cd e2e/bug-15102
+              composer install
+              ../../bin/phpstan analyze
           - script: |
               cd e2e/bug-14724
               composer install
diff --git a/e2e/bug-15102/.gitignore b/e2e/bug-15102/.gitignore
new file mode 100644
index 0000000000..3a9875b460
--- /dev/null
+++ b/e2e/bug-15102/.gitignore
@@ -0,0 +1,2 @@
+/vendor/
+composer.lock
diff --git a/e2e/bug-15102/bootstrap.php b/e2e/bug-15102/bootstrap.php
new file mode 100644
index 0000000000..b78f0408c9
--- /dev/null
+++ b/e2e/bug-15102/bootstrap.php
@@ -0,0 +1,30 @@
+<?php declare(strict_types = 1);
+
+// The shape typo3/class-alias-loader creates: the project's own Composer loader is taken
+// out of the spl_autoload queue and a wrapper takes its place, resolving legacy names
+// through class_alias() and delegating everything else. The wrapper is not a ClassLoader
+// instance, so the project has no Composer entry left in the queue.
+$composerLoader = require __DIR__ . '/vendor/autoload.php';
+spl_autoload_unregister([$composerLoader, 'loadClass']);
+
+final class AliasLoader
+{
+
+	public function __construct(private Composer\Autoload\ClassLoader $wrapped)
+	{
+	}
+
+	public function loadClass(string $class): void
+	{
+		if ($class === 'Legacy\\Validate') {
+			class_alias(Modern\Validate::class, 'Legacy\\Validate');
+
+			return;
+		}
+
+		$this->wrapped->loadClass($class);
+	}
+
+}
+
+spl_autoload_register([new AliasLoader($composerLoader), 'loadClass']);
diff --git a/e2e/bug-15102/composer.json b/e2e/bug-15102/composer.json
new file mode 100644
index 0000000000..4865ec6b07
--- /dev/null
+++ b/e2e/bug-15102/composer.json
@@ -0,0 +1,7 @@
+{
+	"autoload": {
+		"psr-4": {
+			"Modern\\": "src/"
+		}
+	}
+}
diff --git a/e2e/bug-15102/phpstan.dist.neon b/e2e/bug-15102/phpstan.dist.neon
new file mode 100644
index 0000000000..d802ef8169
--- /dev/null
+++ b/e2e/bug-15102/phpstan.dist.neon
@@ -0,0 +1,8 @@
+parameters:
+	level: 8
+	bootstrapFiles:
+		- bootstrap.php
+	paths:
+		- test.php
+		- src
+		- stub
diff --git a/e2e/bug-15102/src/Validate.php b/e2e/bug-15102/src/Validate.php
new file mode 100644
index 0000000000..897b1df0dd
--- /dev/null
+++ b/e2e/bug-15102/src/Validate.php
@@ -0,0 +1,12 @@
+<?php declare(strict_types = 1);
+
+namespace Modern;
+
+class Validate
+{
+
+	public function doFoo(): void
+	{
+	}
+
+}
diff --git a/e2e/bug-15102/stub/Validate.php b/e2e/bug-15102/stub/Validate.php
new file mode 100644
index 0000000000..fbed0ef690
--- /dev/null
+++ b/e2e/bug-15102/stub/Validate.php
@@ -0,0 +1,13 @@
+<?php declare(strict_types = 1);
+
+namespace Legacy;
+
+// An IDE-only stub, never loaded at runtime: it declares the legacy name as a plain class
+// so editors resolve it. At runtime the name is a class_alias to Modern\Validate, which is
+// the declaration PHPStan has to use.
+die('never loaded at runtime');
+
+class Validate
+{
+
+}
diff --git a/e2e/bug-15102/test.php b/e2e/bug-15102/test.php
new file mode 100644
index 0000000000..ad050880ff
--- /dev/null
+++ b/e2e/bug-15102/test.php
@@ -0,0 +1,5 @@
+<?php declare(strict_types = 1);
+
+function (Legacy\Validate $validate): void {
+	$validate->doFoo();
+};
diff --git a/src/autoloadFunctions.php b/src/autoloadFunctions.php
index 617d7f7ee5..17bcce1dcc 100644
--- a/src/autoloadFunctions.php
+++ b/src/autoloadFunctions.php
@@ -3,10 +3,12 @@
 namespace PHPStan;
 
 use Composer\Autoload\ClassLoader;
+use function array_shift;
 use function count;
 use function get_class;
 use function is_array;
 use function is_object;
+use function spl_object_id;
 
 /**
  * Autoloaders that were registered *after* Composer's class loader in the
@@ -33,6 +35,19 @@ function autoloadFunctionsPrependedToComposer(): array // phpcs:ignore Squiz.Fun
 	return $GLOBALS['__phpstanAutoloadFunctionsPrependedToComposer'] ?? [];
 }
 
+/**
+ * Whether the spl_autoload entry is a Composer ClassLoader's loadClass() callable.
+ *
+ * @param mixed $autoloadFunction
+ */
+function isComposerClassLoader($autoloadFunction): bool // phpcs:ignore Squiz.Functions.GlobalFunction.Found
+{
+	return is_array($autoloadFunction)
+		&& count($autoloadFunction) > 0
+		&& is_object($autoloadFunction[0])
+		&& get_class($autoloadFunction[0]) === ClassLoader::class;
+}
+
 /**
  * Splits the autoload functions registered while loading Composer's autoloader
  * and the bootstrap files into those registered before and after Composer's own
@@ -40,6 +55,14 @@ function autoloadFunctionsPrependedToComposer(): array // phpcs:ignore Squiz.Fun
  * same order relative to Composer as PHP does at runtime, instead of always
  * invoking them before (or after) the static Composer source locators.
  *
+ * When no Composer ClassLoader instance is left in the queue there is no
+ * boundary to split on: a bootstrap file has taken Composer's place, so its
+ * loader carries Composer's runtime priority and belongs before the static
+ * source locators. Bucketing those as appended would demote a loader that
+ * actually resolves the class first, which is what
+ * https://github.com/phpstan/phpstan/issues/15102 reported for
+ * typo3/class-alias-loader.
+ *
  * @param list<mixed>|false $autoloadFunctionsBefore
  * @param list<mixed>|false $autoloadFunctionsAfter
  * @return array{prepended: list<mixed>, appended: list<mixed>}
@@ -53,16 +76,28 @@ function collectNewAutoloadFunctions($autoloadFunctionsBefore, $autoloadFunction
 		return ['prepended' => $prepended, 'appended' => $appended];
 	}
 
+	// The split has to happen at the *analysed project's* class loader. PHPStan's own
+	// loader is a ClassLoader as well and is always registered first - before any project
+	// code runs - so searching the queue for the first ClassLoader instance would make
+	// every bootstrap-registered autoloader look like it came after Composer, and demote
+	// it below the static source locators.
+	$classLoaderIds = [];
+	foreach ($autoloadFunctionsBefore as $before) {
+		if (isComposerClassLoader($before)) {
+			$classLoaderIds[] = spl_object_id($before[0]);
+		}
+	}
+
+	array_shift($classLoaderIds);
+	$projectLoaderId = $classLoaderIds === [] ? null : $classLoaderIds[count($classLoaderIds) - 1];
+
 	$composerIndex = null;
-	foreach ($autoloadFunctionsAfter as $index => $after) {
-		if (
-			is_array($after)
-			&& count($after) > 0
-			&& is_object($after[0])
-			&& get_class($after[0]) === ClassLoader::class
-		) {
-			$composerIndex = $index;
-			break;
+	if ($projectLoaderId !== null) {
+		foreach ($autoloadFunctionsAfter as $index => $after) {
+			if (isComposerClassLoader($after) && spl_object_id($after[0]) === $projectLoaderId) {
+				$composerIndex = $index;
+				break;
+			}
 		}
 	}
 
@@ -85,7 +120,7 @@ function collectNewAutoloadFunctions($autoloadFunctionsBefore, $autoloadFunction
 			}
 		}
 
-		if ($composerIndex !== null && $index < $composerIndex) {
+		if ($composerIndex === null || $index < $composerIndex) {
 			$prepended[] = $after;
 		} else {
 			$appended[] = $after;
diff --git a/tests/PHPStan/CollectNewAutoloadFunctionsTest.php b/tests/PHPStan/CollectNewAutoloadFunctionsTest.php
index 9e2c18f551..4f83636c71 100644
--- a/tests/PHPStan/CollectNewAutoloadFunctionsTest.php
+++ b/tests/PHPStan/CollectNewAutoloadFunctionsTest.php
@@ -5,6 +5,12 @@
 use Composer\Autoload\ClassLoader;
 use PHPUnit\Framework\TestCase;
 
+/**
+ * The before-snapshot always starts with PHPStan's own Composer ClassLoader - bin/phpstan
+ * requires its own autoloader long before any project code runs - so every case here models
+ * that entry. Which loader the split happens at is the whole point: see
+ * https://github.com/phpstan/phpstan/issues/15102
+ */
 class CollectNewAutoloadFunctionsTest extends TestCase
 {
 
@@ -23,16 +29,17 @@ public function testFalseInputsYieldEmptyResult(): void
 		);
 	}
 
-	public function testAutoloadersAreSplitByComposerPosition(): void
+	public function testAutoloadersAreSplitByTheProjectsComposerPosition(): void
 	{
+		$phpstanOwn = [new ClassLoader(), 'loadClass'];
+		$project = [new ClassLoader(), 'loadClass'];
 		$prepended = static function (string $class): void {
 		};
-		$composer = new ClassLoader();
 		$appended = static function (string $class): void {
 		};
 
-		$before = [];
-		$after = [$prepended, [$composer, 'loadClass'], $appended];
+		$before = [$phpstanOwn, $project];
+		$after = [$phpstanOwn, $prepended, $project, $appended];
 
 		$result = collectNewAutoloadFunctions($before, $after);
 
@@ -40,29 +47,68 @@ public function testAutoloadersAreSplitByComposerPosition(): void
 		$this->assertSame([$appended], $result['appended']);
 	}
 
-	public function testWithoutComposerEverythingIsAppended(): void
+	/**
+	 * PHPStan's own loader must never be the boundary. It is registered before anything
+	 * else, so splitting on the first ClassLoader in the queue would classify every
+	 * bootstrap-registered autoloader as "after Composer" and consult it only after the
+	 * static source locators - the regression this test pins.
+	 */
+	public function testPhpstansOwnLoaderIsNotTheBoundary(): void
 	{
-		$first = static function (string $class): void {
+		$phpstanOwn = [new ClassLoader(), 'loadClass'];
+		$bootstrap = static function (string $class): void {
 		};
-		$second = static function (string $class): void {
+
+		$before = [$phpstanOwn];
+		$after = [$phpstanOwn, $bootstrap];
+
+		$result = collectNewAutoloadFunctions($before, $after);
+
+		$this->assertSame([$bootstrap], $result['prepended']);
+		$this->assertSame([], $result['appended']);
+	}
+
+	/**
+	 * The shape typo3/class-alias-loader creates: the bootstrap unregisters
+	 * [$composerLoader, 'loadClass'] and registers a wrapper that delegates to it. With the
+	 * project's loader gone from the queue there is no boundary left, and the wrapper holds
+	 * the priority Composer had, so everything collected is consulted first.
+	 */
+	public function testProjectLoaderReplacedByABootstrapWrapperIsPrepended(): void
+	{
+		$phpstanOwn = [new ClassLoader(), 'loadClass'];
+		$project = [new ClassLoader(), 'loadClass'];
+		$replacement = new class {
+
+			public function loadClass(string $class): void
+			{
+			}
+
+		};
+		$wrapper = [$replacement, 'loadClass'];
+		$bootstrap = static function (string $class): void {
 		};
 
-		$result = collectNewAutoloadFunctions([], [$first, $second]);
+		$before = [$phpstanOwn, $project];
+		$after = [$phpstanOwn, $wrapper, $bootstrap];
 
-		$this->assertSame([], $result['prepended']);
-		$this->assertSame([$first, $second], $result['appended']);
+		$result = collectNewAutoloadFunctions($before, $after);
+
+		$this->assertSame([$wrapper, $bootstrap], $result['prepended']);
+		$this->assertSame([], $result['appended']);
 	}
 
 	public function testComposerAndPharAutoloaderAndPreexistingAreExcluded(): void
 	{
+		$phpstanOwn = [new ClassLoader(), 'loadClass'];
 		$preexisting = static function (string $class): void {
 		};
-		$composer = new ClassLoader();
+		$project = [new ClassLoader(), 'loadClass'];
 		$bootstrap = static function (string $class): void {
 		};
 
-		$before = [$preexisting];
-		$after = [$preexisting, [$composer, 'loadClass'], ['PHPStan\\PharAutoloader', 'loadClass'], $bootstrap];
+		$before = [$phpstanOwn, $preexisting, $project];
+		$after = [$phpstanOwn, $preexisting, $project, ['PHPStan\\PharAutoloader', 'loadClass'], $bootstrap];
 
 		$result = collectNewAutoloadFunctions($before, $after);
 
@@ -72,16 +118,17 @@ public function testComposerAndPharAutoloaderAndPreexistingAreExcluded(): void
 
 	public function testPreexistingAutoloaderBeforeComposerIsNotReported(): void
 	{
+		$phpstanOwn = [new ClassLoader(), 'loadClass'];
 		$preexisting = static function (string $class): void {
 		};
-		$composer = new ClassLoader();
+		$project = [new ClassLoader(), 'loadClass'];
 		$prependedBootstrap = static function (string $class): void {
 		};
 
 		// $preexisting was registered before PHPStan loaded the project - it must
 		// be ignored even though it sits before Composer in the queue.
-		$before = [$preexisting];
-		$after = [$preexisting, $prependedBootstrap, [$composer, 'loadClass']];
+		$before = [$phpstanOwn, $preexisting, $project];
+		$after = [$phpstanOwn, $preexisting, $prependedBootstrap, $project];
 
 		$result = collectNewAutoloadFunctions($before, $after);