RishavTiwari25 opened a new issue, #26243:
URL: https://github.com/apache/datafusion/issues/26243
### Describe the bug
`array_concat` / `array_cat` fails to plan when one of the arguments is a
plain `NULL` literal, and the error message names the element type of the
*other* argument:
```sql
> SELECT array_concat([1, 2], NULL);
Error during planning: array_concat does not support type Int64
> SELECT array_cat(NULL, [1, 2]);
Error during planning: array_concat does not support type Int64
```
The same calls work when the `NULL` is typed (`NULL::int[]`) or when a
`NULL` appears inside a list, so this is only the untyped literal:
```sql
> SELECT array_concat([1, 2], NULL::int[]);
[1, 2]
> SELECT array_concat([1, 2], [NULL]);
[1, 2, NULL]
```
`ArrayConcat::return_type` accepts `DataType::Null` arguments, but
`ArrayConcat::coerce_types` runs every argument through
`coerced_type_with_base_type_only`, whose fallback arm replaces a non-list type
with the base type. The `Null` argument therefore becomes `Int64`, and the
second `return_type` call rejects it.
### To Reproduce
In `datafusion-cli`:
```sql
SELECT array_concat([1, 2], NULL);
SELECT array_cat(NULL, [1, 2]);
SELECT array_concat(['a'], NULL);
```
### Expected behavior
The untyped `NULL` is treated like a `NULL` list of the result type and the
other arguments are returned, matching PostgreSQL (`array_cat(ARRAY[1, 2],
NULL)` is `{1,2}`) and the typed-`NULL` behavior above:
```sql
> SELECT array_concat([1, 2], NULL);
[1, 2]
```
### Additional context
Reproduced on `main` at 6663e4348 (debug build of `datafusion-cli`).
#16349 covered `NULL::int[]` arguments; this is the remaining untyped case.
I would like to work on this.
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