diff --git a/src/Symfony/Component/DependencyInjection/Loader/YamlFileLoader.php b/src/Symfony/Component/DependencyInjection/Loader/YamlFileLoader.php
index c3e697c1b5207..19d919f2eda0e 100644
--- a/src/Symfony/Component/DependencyInjection/Loader/YamlFileLoader.php
+++ b/src/Symfony/Component/DependencyInjection/Loader/YamlFileLoader.php
@@ -315,8 +315,9 @@ private function isUsingShortSyntax(array $service)
     private function parseDefinition($id, $service, $file, array $defaults)
     {
         if (preg_match('/^_[a-zA-Z0-9_]*$/', $id)) {
-            @trigger_error(sprintf('Service names that start with an underscore are deprecated since Symfony 3.3 and will be reserved in 4.0. Rename the "%s" service or define it in XML instead.', $id), E_USER_DEPRECATED);
+            throw new InvalidArgumentException(sprintf('Service names that start with an underscore are reserved. Rename the "%s" service or define it in XML instead.', $id));
         }
+
         if (is_string($service) && 0 === strpos($service, '@')) {
             $public = isset($defaults['public']) ? $defaults['public'] : true;
             $this->container->setAlias($id, new Alias(substr($service, 1), $public));
@@ -344,7 +345,7 @@ private function parseDefinition($id, $service, $file, array $defaults)
 
             foreach ($service as $key => $value) {
                 if (!in_array($key, array('alias', 'public'))) {
-                    @trigger_error(sprintf('The configuration key "%s" is unsupported for the service "%s" which is defined as an alias in "%s". Allowed configuration keys for service aliases are "alias" and "public". The YamlFileLoader will raise an exception in Symfony 4.0, instead of silently ignoring unsupported attributes.', $key, $id, $file), E_USER_DEPRECATED);
+                    throw new InvalidArgumentException(sprintf('The configuration key "%s" is unsupported for the service "%s" which is defined as an alias in "%s". Allowed configuration keys for service aliases are "alias" and "public".', $key, $id, $file));
                 }
             }
 
@@ -740,11 +741,6 @@ private function resolveServices($value, $file, $isParameter = false)
                 $invalidBehavior = ContainerInterface::EXCEPTION_ON_INVALID_REFERENCE;
             }
 
-            if ('=' === substr($value, -1)) {
-                @trigger_error(sprintf('The "=" suffix that used to disable strict references in Symfony 2.x is deprecated since 3.3 and will be unsupported in 4.0. Remove it in "%s".', $value), E_USER_DEPRECATED);
-                $value = substr($value, 0, -1);
-            }
-
             if (null !== $invalidBehavior) {
                 $value = new Reference($value, $invalidBehavior);
             }
@@ -782,7 +778,7 @@ private function loadFromExtensions(array $content)
      */
     private function checkDefinition($id, array $definition, $file)
     {
-        if ($throw = $this->isLoadingInstanceof) {
+        if ($this->isLoadingInstanceof) {
             $keywords = self::$instanceofKeywords;
         } elseif ($throw = isset($definition['resource'])) {
             $keywords = self::$prototypeKeywords;
@@ -792,11 +788,7 @@ private function checkDefinition($id, array $definition, $file)
 
         foreach ($definition as $key => $value) {
             if (!isset($keywords[$key])) {
-                if ($throw) {
-                    throw new InvalidArgumentException(sprintf('The configuration key "%s" is unsupported for definition "%s" in "%s". Allowed configuration keys are "%s".', $key, $id, $file, implode('", "', $keywords)));
-                }
-
-                @trigger_error(sprintf('The configuration key "%s" is unsupported for service definition "%s" in "%s". Allowed configuration keys are "%s". The YamlFileLoader object will raise an exception instead in Symfony 4.0 when detecting an unsupported service configuration key.', $key, $id, $file, implode('", "', $keywords)), E_USER_DEPRECATED);
+                throw new InvalidArgumentException(sprintf('The configuration key "%s" is unsupported for definition "%s" in "%s". Allowed configuration keys are "%s".', $key, $id, $file, implode('", "', $keywords)));
             }
         }
     }
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/yaml/bad_alias.yml b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/yaml/bad_alias.yml
new file mode 100644
index 0000000000000..78975e5092866
--- /dev/null
+++ b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/yaml/bad_alias.yml
@@ -0,0 +1,11 @@
+services:
+    foo:
+        class: stdClass
+        public: false
+
+    bar:
+        alias: foo
+        public: true
+        # keys other than "alias" and "public" are invalid when defining an alias.
+        calls:
+            - [doSomething]
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/yaml/legacy_invalid_definition.yml b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/yaml/bad_keyword.yml
similarity index 100%
rename from src/Symfony/Component/DependencyInjection/Tests/Fixtures/yaml/legacy_invalid_definition.yml
rename to src/Symfony/Component/DependencyInjection/Tests/Fixtures/yaml/bad_keyword.yml
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Loader/YamlFileLoaderTest.php b/src/Symfony/Component/DependencyInjection/Tests/Loader/YamlFileLoaderTest.php
index 8de64d55f4486..037af216157c0 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Loader/YamlFileLoaderTest.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/Loader/YamlFileLoaderTest.php
@@ -510,8 +510,8 @@ public function testInvalidTagsWithDefaults()
     }
 
     /**
-     * @group legacy
-     * @expectedDeprecation Service names that start with an underscore are deprecated since Symfony 3.3 and will be reserved in 4.0. Rename the "_foo" service or define it in XML instead.
+     * @expectedException \Symfony\Component\DependencyInjection\Exception\InvalidArgumentException
+     * @expectedExceptionMessage Service names that start with an underscore are reserved. Rename the "_foo" service or define it in XML instead.
      */
     public function testUnderscoreServiceId()
     {
@@ -633,6 +633,28 @@ public function testEmptyInstanceofThrowsClearException()
         $loader = new YamlFileLoader($container, new FileLocator(self::$fixturesPath.'/yaml'));
         $loader->load('bad_empty_instanceof.yml');
     }
+
+    /**
+     * @expectedException \Symfony\Component\DependencyInjection\Exception\InvalidArgumentException
+     * @expectedExceptionMessageRegExp /^The configuration key "private" is unsupported for definition "bar"/
+     */
+    public function testUnsupportedKeywordThrowsException()
+    {
+        $container = new ContainerBuilder();
+        $loader = new YamlFileLoader($container, new FileLocator(self::$fixturesPath.'/yaml'));
+        $loader->load('bad_keyword.yml');
+    }
+
+    /**
+     * @expectedException \Symfony\Component\DependencyInjection\Exception\InvalidArgumentException
+     * @expectedExceptionMessageRegExp /^The configuration key "calls" is unsupported for the service "bar" which is defined as an alias/
+     */
+    public function testUnsupportedKeywordInServiceAliasThrowsException()
+    {
+        $container = new ContainerBuilder();
+        $loader = new YamlFileLoader($container, new FileLocator(self::$fixturesPath.'/yaml'));
+        $loader->load('bad_alias.yml');
+    }
 }
 
 interface FooInterface