Warning hints at functionality that doesn't exist when using out of scope temporary variable with +=

Temporary variables appear to only be accessible in the scope they were declared in, and not accessible to child containers.
The error message shown when trying to access them using += suggests that they should be accessible, as if ink was searching parent objects, but it seems not to.

The image shows a repro.

When using = the error message reflects how ink actually works - it doesn't search parent objects.