[DependencyInjection] Improve an exception message

src/Symfony/Component/DependencyInjection/Compiler/ResolveClassPass.php

@@ -29,10 +29,13 @@ public function process(ContainerBuilder $container)
             if ($definition->isSynthetic() || null !== $definition->getClass()) {
                 continue;
             }
-            if (preg_match('/^[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*+(?:\\\\[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*+)++$/', $id)) {
+            if (preg_match('/^\\\\?[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*+(?:\\\\[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*+)++$/', $id)) {
                 if ($definition instanceof ChildDefinition && !class_exists($id)) {
                     throw new InvalidArgumentException(sprintf('Service definition "%s" has a parent but no class, and its name looks like a FQCN. Either the class is missing or you want to inherit it from the parent service. To resolve this ambiguity, please rename this service to a non-FQCN (e.g. using dots), or create the missing class.', $id));
                 }
+                if ('\\' === $id[0]) {
+                    throw new InvalidArgumentException(sprintf('Service definition "%s" has no class, and its name looks like a FQCN but it starts with a backslash; remove the leading backslash.', $id));
+                }
                 $definition->setClass($id);
             }
         }

src/Symfony/Component/DependencyInjection/Tests/Compiler/ResolveClassPassTest.php

@@ -15,6 +15,7 @@
 use Symfony\Component\DependencyInjection\ChildDefinition;
 use Symfony\Component\DependencyInjection\Compiler\ResolveClassPass;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
+use Symfony\Component\DependencyInjection\Exception\InvalidArgumentException;
 use Symfony\Component\DependencyInjection\Tests\Fixtures\CaseSensitiveClass;
 
 class ResolveClassPassTest extends TestCase
@@ -58,6 +59,17 @@ public function provideInvalidClassId()
         yield ['\DateTime'];
     }
 
+    public function testWontResolveClassFromClassIdWithLeadingBackslash()
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('Service definition "\App\Some\Service" has no class, and its name looks like a FQCN but it starts with a backslash; remove the leading backslash.');
+
+        $container = new ContainerBuilder();
+        $container->register('\App\Some\Service');
+
+        (new ResolveClassPass())->process($container);
+    }
+
     public function testNonFqcnChildDefinition()
     {
         $container = new ContainerBuilder();