[DI] Cannot override parent with named arguments

[ro0NL]

Q	A
Bug report?	yes
Feature request?	yes
BC Break report?	no
Symfony version	3.x

Given

services:
    AppBundle\Foo: [a, b]
    foo:
        parent: AppBundle\Foo
        arguments: { '$b': b_custom }

I get

ContextErrorException in ResolveDefinitionTemplatesPass.php line 173:
Notice: Undefined variable: index

I checked it out and tried a quick patch

diff --git a/src/Symfony/Component/DependencyInjection/Compiler/ResolveDefinitionTemplatesPass.php b/src/Symfony/Component/DependencyInjection/Compiler/ResolveDefinitionTemplatesPass.php
index 9cc2250..b3856b2 100644
--- a/src/Symfony/Component/DependencyInjection/Compiler/ResolveDefinitionTemplatesPass.php
+++ b/src/Symfony/Component/DependencyInjection/Compiler/ResolveDefinitionTemplatesPass.php
@@ -166,7 +166,9 @@ class ResolveDefinitionTemplatesPass extends AbstractRecursivePass
 
             if (0 === strpos($k, 'index_')) {
                 $index = (int) substr($k, strlen('index_'));
-            } elseif (0 !== strpos($k, '$')) {
+            } elseif (0 === strpos($k, '$')) {
+                $index = $k;
+            } else {
                 throw new RuntimeException(sprintf('Invalid argument key "%s" found.', $k));
             }

No luck

OutOfBoundsException in Definition.php line 214:
Service "foo": The argument "$b" doesn't exist.

Kind of a feature request, but certainly an expected one if you ask me.

[nicolas-grekas]

Should be fixed by #22358