Fix the behaviour of combining @data and @singleton

[rayanht]

Describe the bug

When combining @singleton with @data, @singleton should come before @data. Combining them the other way around will work in most cases but is not thoroughly tested and relies on assumptions that might not hold.

Expected behavior
There should be a single correct way of emulating this behaviour. We should raise an immediately actionable error when they are combined in a way that isn't guaranteed to function.

[suneel944]

why this has been reopened? any issue persisting?