zanmato1984 opened a new issue, #50869:
URL: https://github.com/apache/arrow/issues/50869
### Describe the bug, including details regarding any error messages,
version, and platform.
On current `main` (`e611f48081`), expression binding can incorrectly treat
mixed decimal arguments to `coalesce` as an exact kernel match. This bypasses
the decimal normalization in `CoalesceFunction::DispatchBest` and leaves the
bound expression without the required cast to a common decimal type.
For a schema containing:
```text
left: decimal128(3, 2)
right: decimal128(4, 3)
```
binding:
```cpp
call("coalesce", {field_ref("left"), field_ref("right")})
```
currently produces:
```text
coalesce(left, right)
```
instead of:
```text
coalesce(cast(left, decimal128(4, 3)), right)
```
With `left = [1.23, null]` and `right = [null, 2.345]`, executing the
incorrectly bound expression fails with:
```text
Type error: All types must be compatible, expected: decimal128(3, 2), but
got: decimal128(4, 3)
```
The expected result is a `decimal128(4, 3)` array containing `[1.230,
2.345]`.
Calling `DispatchBest` directly already normalizes these arguments
correctly. The problem is that expression binding tries `DispatchExact` first,
and the broad decimal varargs signature accepts the mixed concrete decimal
types before `DispatchBest` can insert the cast.
This appears to be another instance of the exact-dispatch constraint problem
addressed more generally by #47287 and the `MatchConstraint` mechanism
introduced in #47297.
### Component(s)
C++
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