The Avg aggregate should only automatically resolves its output field to FloatField on numeric sources

[Simon Charette]

Before #24649 was fixed it was simply not possible to use the Avg function on non-numeric sources. The *good enough* solution was to allow use of the output_field kwarg and document it should be used when averaging DurationField.

As DurationField uses different underlying datatype for its storage (bigint, interval, ...) averaging on such fields ​might require or not an explicit output_field and can be a source of confusion.

As the SQL AVG function only defaults to using floating value datatype as a return value when numeric input is used I suggest adjusting the Avg expression's output_field resolution to only use FloatField for default when its source field is an instance of one of Django's numerical field classes. This would lift the requirement of explicitly specifying an output_field when averaging DurationField on PostgreSQL.

[Simon Charette]

​https://github.com/django/django/pull/6413

[Simon Charette <charettes@…>]

In a6074e89:

Fixed #26458 -- Based Avg's default output_field resolution on its source field type.

Thanks Tim for the review and Josh for the input.