Fix Nullable(X)->Y cross-type ALTER MODIFY COLUMN

PR ClickHouse#84770 generates _CAST(ifNull(col, default), TargetType).
When source and target base types differ (e.g. Nullable(UInt8) -> String),
ifNull throws NO_COMMON_TYPE because getLeastSupertype({UInt8, String}) fails.

Fix: swap to ifNull(_CAST(col, Nullable(TargetType)), _CAST(default, TargetType)).

Ref: ClickHouse#84770, ClickHouse#5985
Signed-off-by: Daniel Q. Kim <daniel.kim@altinity.com>

Author: Daniel Q. Kim

src/Interpreters/inplaceBlockConversions.cpp

@@ -166,10 +166,18 @@ ASTPtr convertRequiredExpressions(Block & block, const NamesAndTypesList & requi
                     "Please specify `DEFAULT` expression in ALTER MODIFY COLUMN statement",
                     required_column.name, column_in_block.type->getName(), required_column.type->getName());
 
-            auto convert_func = makeASTFunction("_CAST",
-                makeASTFunction("ifNull", std::make_shared<ASTIdentifier>(required_column.name), default_value),
+            /// Cast the nullable column to Nullable(TargetType) first, then strip NULLs with ifNull.
+            /// This handles cross-type conversion (e.g. Nullable(UInt8) -> String) correctly,
+            /// because _CAST(Nullable(UInt8), 'Nullable(String)') -> Nullable(String),
+            /// and ifNull(Nullable(String), String) trivially resolves to String.
+            auto nullable_target_type_name = "Nullable(" + required_column.type->getName() + ")";
+            auto cast_col = makeASTFunction("_CAST",
+                std::make_shared<ASTIdentifier>(required_column.name),
+                std::make_shared<ASTLiteral>(nullable_target_type_name));
+            auto cast_default = makeASTFunction("_CAST",
+                default_value,
                 std::make_shared<ASTLiteral>(required_column.type->getName()));
-
+            auto convert_func = makeASTFunction("ifNull", std::move(cast_col), std::move(cast_default));
             conversion_expr_list->children.emplace_back(setAlias(convert_func, required_column.name));
             continue;
         }

tests/queries/0_stateless/03575_modify_column_null_to_default_cross_type.reference

@@ -0,0 +1,6 @@
+\N	a
+42	b
+	a
+42	b
+	a
+42	b

tests/queries/0_stateless/03575_modify_column_null_to_default_cross_type.sql

@@ -0,0 +1,14 @@
+-- Tags: no-random-settings, no-random-merge-tree-settings
+
+-- Test cross-type Nullable(X) -> Y conversion (e.g. Nullable(UInt8) -> String)
+-- Follow-up to PR #84770
+
+DROP TABLE IF EXISTS nullable_cross_type_test;
+CREATE TABLE nullable_cross_type_test (x Nullable(UInt8), y String) ORDER BY tuple();
+INSERT INTO nullable_cross_type_test VALUES (NULL, 'a'), (42, 'b');
+SELECT * FROM nullable_cross_type_test ORDER BY y;
+ALTER TABLE nullable_cross_type_test MODIFY COLUMN x String DEFAULT '';
+SELECT * FROM nullable_cross_type_test ORDER BY y;
+OPTIMIZE TABLE nullable_cross_type_test FINAL;
+SELECT * FROM nullable_cross_type_test ORDER BY y;
+DROP TABLE nullable_cross_type_test;