Tagged services - misleading exception

Updated for message from #21.

not sure what happened in #37

The last submitted patch, 38: 2652694-38.patch, failed testing. View results →

Random failure

IMO the change in the exception message from #37 to #38
- does not implement $interface, or its namespace cannot be resolved.
+ does not have an existing class $interface.
is restoring some confusion to the dev/reader, i.e. it does little to inform that the provided class: for the collected service does not exist. The $interface variable will be generated based on the service collector's definition and not the collected service.

Even more, the actual change in code could focus explicitly on this case, e.g.:

+++ b/core/lib/Drupal/Core/DependencyInjection/Compiler/TaggedHandl
ersPass.php
@@ -178,6 +178,9 @@ protected function processServiceCollectorPass(
array $pass, $consumer_id, Contai
     foreach ($this->tagCache[$tag] ?? [] as $id => $attributes) {
       // Validate the interface.
       $handler = $container->getDefinition($id);
+      if (class_exists($handler->getClass()) === FALSE) {
+        throw new LogicException("Service '$id' for consumer '$con
sumer_id' does not have a corresponding existing class " . $handler
->getClass());
+      }
       if (!is_subclass_of($handler->getClass(), $interface)) {
         throw new LogicException("Service '$id' for consumer '$con
sumer_id' does not implement $interface.");
       }

That would mean a small change in the first one of the new tests to capture the new exception for non-existing classes.
And a small change in the second test, if we stick to the original exception message (Service '$id' for consumer '$consumer_id' does not implement $interface.