[C++] coalesce expressions fail for mixed dictionary types #55404

Describe the bug, including details regarding any error messages, version, and platform.

A bound coalesce expression fails at execution when dictionary arguments have compatible value types but differ in index type, value type, or the ordered flag.

For example, consider coalesce(x, y) with these dictionary-encoded inputs:

x: dictionary<values=int32, indices=int8, ordered=0>
   values: [10, null, 30, null, null]
y: dictionary<values=int32, indices=int16, ordered=0>
   values: [null, 20, 40, null, null]

Expression binding succeeds, but execution returns:

Type error: All types must be compatible,
expected: dictionary<values=int32, indices=int8, ordered=0>,
but got: dictionary<values=int32, indices=int16, ordered=0>

The dictionary kernel matches these arguments during exact dispatch without checking full type equality. This prevents expression binding from falling back to best dispatch and inserting the necessary casts, although the kernel requires identical types at execution.

The expected result is an int32 array containing [10, 20, 30, null, null]. Compatible mixed dictionary arguments should follow the existing decoding and type-promotion path. Arguments with identical dictionary types should retain dictionary-encoded execution.

Component(s)

C++