Clearly gate sliding SUM(DISTINCT) type support (#22866)

## Which issue does this PR close?

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- Closes #22820.

## Rationale for this change

Sliding `SUM(DISTINCT)` only supports `Int64`, but it was routed through
the wider `SUM` type dispatch path. This made unsupported types fail
with a less clear accumulator error.

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## What changes are included in this PR?

This PR adds an explicit `Int64` gate for sliding `SUM(DISTINCT)`.

Unsupported types now return a clear feature error that names the
operation and type. The existing `Int64` path is unchanged.

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## Are these changes tested?

Yes

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## Are there any user-facing changes?

No public API change

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Author: kumarUjjawal

datafusion/functions-aggregate/src/sum.rs

@@ -303,13 +303,18 @@ impl AggregateUDFImpl for Sum {
         args: AccumulatorArgs,
     ) -> Result<Box<dyn Accumulator>> {
         if args.is_distinct {
-            // distinct path: use our sliding‐window distinct‐sum
-            macro_rules! helper_distinct {
-                ($t:ty, $dt:expr) => {
-                    Ok(Box::new(SlidingDistinctSumAccumulator::try_new(&$dt)?))
-                };
+            // distinct path: [`SlidingDistinctSumAccumulator`] only implements
+            // Int64, so gate the supported type here rather than dispatching
+            // through `downcast_sum!`, which accepts every SUM type
+            match args.return_field.data_type() {
+                DataType::Int64 => Ok(Box::new(SlidingDistinctSumAccumulator::try_new(
+                    &DataType::Int64,
+                )?)),
+                _ => not_impl_err!(
+                    "SUM(DISTINCT) over sliding window frames is only supported for Int64, got {}",
+                    args.expr_fields[0].data_type()
+                ),
             }
-            downcast_sum!(args, helper_distinct)
         } else {
             // non‐distinct path: existing sliding sum
             macro_rules! helper {
@@ -525,7 +530,9 @@ impl SlidingDistinctSumAccumulator {
     pub fn try_new(data_type: &DataType) -> Result<Self> {
         // TODO support other numeric types
         if *data_type != DataType::Int64 {
-            return exec_err!("SlidingDistinctSumAccumulator only supports Int64");
+            return exec_err!(
+                "SlidingDistinctSumAccumulator only supports Int64, got {data_type}"
+            );
         }
         Ok(Self {
             counts: HashMap::default(),

datafusion/sqllogictest/test_files/window.slt

@@ -5981,6 +5981,41 @@ FROM table_distinct_sum_nulls;
 5 5
 
 
+# SUM(DISTINCT) over sliding (bounded) window frames is only implemented
+# for Int64. Other SUM-supported input types must fail with a clear
+# capability error instead of an accumulator-internal one.
+statement ok
+CREATE TABLE table_distinct_sum_types(ts INT, f DOUBLE, d DECIMAL(10, 2)) AS VALUES
+  (1, 1.5, 1.50), (2, 2.5, 2.50), (3, 1.5, 1.50);
+
+query error DataFusion error: This feature is not implemented: SUM\(DISTINCT\) over sliding window frames is only supported for Int64, got Float64
+SELECT SUM(DISTINCT f) OVER (
+  ORDER BY ts ROWS BETWEEN 1 PRECEDING AND CURRENT ROW
+) FROM table_distinct_sum_types;
+
+query error DataFusion error: This feature is not implemented: SUM\(DISTINCT\) over sliding window frames is only supported for Int64, got Decimal128\(10, 2\)
+SELECT SUM(DISTINCT d) OVER (
+  ORDER BY ts ROWS BETWEEN 1 PRECEDING AND CURRENT ROW
+) FROM table_distinct_sum_types;
+
+query error DataFusion error: This feature is not implemented: SUM\(DISTINCT\) over sliding window frames is only supported for Int64, got UInt64
+SELECT SUM(DISTINCT arrow_cast(ts, 'UInt64')) OVER (
+  ORDER BY ts ROWS BETWEEN 1 PRECEDING AND CURRENT ROW
+) FROM table_distinct_sum_types;
+
+# Unbounded frames take the regular distinct-sum path and keep
+# supporting all SUM input types.
+query R
+SELECT SUM(DISTINCT f) OVER (ORDER BY ts) FROM table_distinct_sum_types;
+----
+1.5
+4
+4
+
+statement ok
+DROP TABLE table_distinct_sum_types;
+
+
 # FILTER clause with window functions
 
 # Verify FILTER clause with non-aggregate window functions fails with a clear message