The  markpublic_pass  compiler pass assumes that every service alias points directly to a service definition.

Symfony permits aliases to reference other aliases. When an extension registers an alias chain such as:

extension.cache_interface:

    alias: text_formatter.cache

where text_formatter.cache is itself an alias, container compilation fails with:

Undefined array key "text_formatter.cache"

Call to a member function isPublic() on null

This was encountered while testing the phpBB Pages extension on phpBB 4. Pages decorates text_formatter.cache and retains the canonical alias so that other extensions can compose additional decorators.

phpBB 3.3.x is unaffected because it does not register markpublic_pass, but other extensions ported to phpBB 4 could trigger the same failure.

The compiler pass should resolve aliases recursively through Symfony’s container API:

$container->findDefinition((string) $alias)

This allows direct and chained aliases to be handled consistently while preserving the existing behavior for public and explicitly private definitions.