sql/schemachanger: Support enum types for DSC impl of ALTER COLUMN ..…

… ALTER TYPE

Previously, attempts to alter a column type using an enum were blocked
due to failures during backfill, which would result in the error 'cannot
resolve types in DistSQL by name.' This change resolves that issue by
ensuring that all type names are resolved when populating the USING
expression for the alter

Epic: CRDB-25314
Closes cockroachdb#132936
Release note: none

pkg/sql/logictest/testdata/logic_test/enums

@@ -1090,7 +1090,7 @@ statement ok
 CREATE TABLE enum_data_type (x STRING);
 INSERT INTO enum_data_type VALUES ('notagreeting')
 
-statement error pq: invalid input value for enum greeting: "notagreeting"
+statement error pq: .*invalid input value for enum greeting: "notagreeting"
 ALTER TABLE enum_data_type ALTER COLUMN x SET DATA TYPE greeting USING x::greeting
 
 statement ok

pkg/sql/schemachanger/scbuild/internal/scbuildstmt/alter_table_alter_column_type.go

@@ -284,14 +284,6 @@ func handleGeneralColumnConversion(
 			"old active version; ALTER COLUMN TYPE requires backfill. Reverting to legacy handling"))
 	}
 
-	// TODO(#132936): Not yet supported in the DSC. Throwing an error to trigger
-	// fallback to legacy.
-	if newColType.Type.Family() == types.EnumFamily {
-		panic(scerrors.NotImplementedErrorf(t,
-			"backfilling during ALTER COLUMN TYPE for an enum column "+
-				"type is not supported"))
-	}
-
 	// Generate the ID of the new column we are adding.
 	newColID := b.NextTableColumnID(tbl)
 	newColType.ColumnID = newColID
@@ -447,7 +439,7 @@ func getComputeExpressionForBackfill(
 		return
 	}
 
-	_, _, _, err = schemaexpr.DequalifyAndValidateExprImpl(b, expr, newColType.Type,
+	typedExpr, _, _, err := schemaexpr.DequalifyAndValidateExprImpl(b, expr, newColType.Type,
 		tree.AlterColumnTypeUsingExpr, b.SemaCtx(), volatility.Volatile, tn, b.ClusterSettings().Version.ActiveVersion(b),
 		func() colinfo.ResultColumns {
 			return getNonDropResultColumns(b, tableID)
@@ -456,6 +448,14 @@ func getComputeExpressionForBackfill(
 			return columnLookupFn(b, tableID, columnName)
 		},
 	)
+	if err != nil {
+		return
+	}
+
+	// Return the updated expression from DequalifyAndValidateExprImpl. For expressions
+	// involving user-defined types like enums, the types will be resolved to ensure
+	// they can be used during backfill.
+	expr, err = parser.ParseExpr(typedExpr)
 	return
 }