SQL: Compound aggregation

[gramian]

In the case of aggregating a single field or literal, a compounding aggregation causes an error, while aggregating record-wise allows an aggregation on top. Here is an example:

SELECT sum(unionall(1,2,3,4)); -- works
SELECT sum(unionall([1,2,3,4])); -- errors

Specifically the error is Cannot calculate an aggregate function of another aggregate function. I understand this limitation conceptually as a type-change is induced by a single argument aggregation, but on the other hand a function should be able to operate on a legal argument with disregard on how it was obtained.

This may also just resolve with #1037 , since:

SELECT sum(first((SELECT unionall([1,2,3,4]) as x)).x)

works.

I am just asking this here separately due to specific error message Cannot calculate an aggregate function of another aggregate function, when should this error (actually?) be triggered?

Thank you