[Sqlalchemy-tickets] Issue #4324: array_agg ignores keyword type_ hint for postgres dialect, failing for a nested array (zzzeek/sqlalchemy)

[Dmitry N. <iss...@bi...>]

New issue 4324: array_agg ignores keyword type_ hint for postgres dialect, failing for a nested array
https://bitbucket.org/zzzeek/sqlalchemy/issues/4324/array_agg-ignores-keyword-type_-hint-for

Dmitry Nikonov:

Hello.

This issue is quite similar to the #4107 but about the [postgresql dialect version](https://bitbucket.org/zzzeek/sqlalchemy/src/master/lib/sqlalchemy/dialects/postgresql/ext.py#lines-209) of the `array_agg`.  I suppose it should be fixed in the same manner.

Code to reproduce (Python 3.7.0, SQLAlchemy 1.2.11):
```python
from sqlalchemy import *
from sqlalchemy.dialects import postgresql

col = column('foo', postgresql.ARRAY(Integer))

function = func.array_agg(col, type_=ARRAY(Integer))  # Works fine

function = func.array_agg(col, type_=postgresql.ARRAY(Integer))  # Works fine too

function = postgresql.array_agg(col, type_=ARRAY(Integer))  # Fails with the trace shown below

function = postgresql.array_agg(col, type_=postgresql.ARRAY(Integer))  # Fails with the same trace
```

Stack trace:
```
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-8-841c5b8114ad> in <module>()
----> 1 function = postgresql.array_agg(col, type_=postgresql.ARRAY(Integer))

/usr/lib/python3.7/site-packages/sqlalchemy/dialects/postgresql/ext.py in array_agg(*arg, **kw)
    215 
    216     """
--> 217     kw['type_'] = ARRAY(functions._type_from_args(arg))
    218     return functions.func.array_agg(*arg, **kw)

/usr/lib/python3.7/site-packages/sqlalchemy/dialects/postgresql/array.py in __init__(self, item_type, as_tuple, dimensions, zero_indexes)
    229         """
    230         if isinstance(item_type, ARRAY):
--> 231             raise ValueError("Do not nest ARRAY types; ARRAY(basetype) "
    232                              "handles multi-dimensional arrays of basetype")
    233         if isinstance(item_type, type):

ValueError: Do not nest ARRAY types; ARRAY(basetype) handles multi-dimensional arrays of basetype

```