Regression: Union of null with non-null (Gopal V, reviewed by Sergey Shelukhin)
hive> select null as c1 union select 1 as c2;
FAILED: SemanticException Schema of both sides of union should match: Column c1 is of type void on first table and type int on second table. Cannot tell the position of null AST.
This looks related to HIVE-14251 which removed certain cases from the UNION type coercion operations.