Spurious cast to Uint8 inserted when coalesce is used in a context of a join

[milevin]

Describe the bug

Following is a valid (as far as I can tell) SQL that blows up in DataFusion:

DataFusion CLI v20.0.0
❯ with customer as (
  select 11111 as c_custkey
), cust as (
  select case when c_custkey = 0 then null else c_custkey end as c_custkey from customer
)
select coalesce(a.c_custkey, b.c_custkey) as custkey
from cust a inner join cust b
on a.c_custkey = b.c_custkey;
Arrow error: Cast error: Can't cast value 11111 to type UInt8

The culprit seems to be a spurious cast to UInt8 that gets inserted in coalesce arguments:

❯ explain with customer as (
  select 11111 as c_custkey
), cust as (
  select case when c_custkey = 0 then null else c_custkey end as c_custkey from customer
)
select coalesce(a.c_custkey, b.c_custkey) as custkey
from cust a inner join cust b
on a.c_custkey = b.c_custkey;
+---------------+----------------------------------------------------------------------------------------------------------------------------------------------+
| plan_type     | plan                                                                                                                                         |
+---------------+----------------------------------------------------------------------------------------------------------------------------------------------+
| logical_plan  | Projection: coalesce(CAST(a.c_custkey AS UInt8), CAST(b.c_custkey AS UInt8)) AS custkey                                                      |
|               |   Inner Join: a.c_custkey = b.c_custkey                                                                                                      |
|               |     SubqueryAlias: a                                                                                                                         |
|               |       SubqueryAlias: cust                                                                                                                    |
|               |         Projection: CASE WHEN customer.c_custkey = Int64(0) THEN Int64(NULL) ELSE customer.c_custkey END AS c_custkey                        |
|               |           SubqueryAlias: customer                                                                                                            |
|               |             Projection: Int64(11111) AS c_custkey                                                                                            |
|               |               EmptyRelation                                                                                                                  |
|               |     SubqueryAlias: b                                                                                                                         |
|               |       SubqueryAlias: cust                                                                                                                    |
|               |         Projection: CASE WHEN customer.c_custkey = Int64(0) THEN Int64(NULL) ELSE customer.c_custkey END AS c_custkey                        |
|               |           SubqueryAlias: customer                                                                                                            |
|               |             Projection: Int64(11111) AS c_custkey                                                                                            |
|               |               EmptyRelation                                                                                                                  |
...

To Reproduce

See above repro in the CLI

Expected behavior

The query above is expected to succeed

Additional context

Any simplification to the above query I tried (e.g. removing join, case, or coalesce) makes the issue go away.

E.g. here is a query where b.custkey is replaced by 0 in the coalesce call:

❯ with customer as (
  select 11111 as c_custkey
), cust as (
  select case when c_custkey = 0 then null else c_custkey end as c_custkey from customer
)
select coalesce(a.c_custkey, 0) as custkey
from cust a inner join cust b
on a.c_custkey = b.c_custkey;
+---------+
| custkey |
+---------+
| 11111   |
+---------+
1 row in set. Query took 0.021 seconds.