linhongliu-db opened a new pull request, #59806:
URL: https://github.com/apache/spark/pull/59806

   ### What changes were proposed in this pull request?
   
   Fix `date_add` and `date_sub` to preserve a foldable NULL string day count 
during
   type coercion. Keep the cast result boxed and construct an INT literal with
   `Literal.create`, so NULL does not become zero.
   
   ### Why are the changes needed?
   
   With ANSI mode disabled, string day counts are evaluated during analysis. 
Unboxing
   a NULL cast result with `asInstanceOf[Int]` produces zero, causing these 
functions
   to return the input date instead of NULL.
   
   ### Does this PR introduce _any_ user-facing change?
   
   Yes, a bug fix. For example:
   
   ```sql
   SET spark.sql.ansi.enabled=false;
   SELECT date_add(DATE '2020-01-01', CAST(NULL AS STRING)),
          date_sub(DATE '2020-01-01', CAST(NULL AS STRING));
   ```
   
   Previously this returned `2020-01-01, 2020-01-01`; it now returns `NULL, 
NULL`.
   Valid non-null string day counts and invalid-string errors are unchanged.
   
   ### How was this patch tested?
   
   Added coercion and SQL regression coverage for direct NULL strings, foldable
   conditional/cast expressions, ANSI mode on and off, valid string day counts, 
and
   invalid-string errors. The new coercion test fails without the fix because 
the
   day-count literal is `0` instead of NULL.
   
   Passed on JDK 17:
   
   ```bash
   build/sbt 'catalyst/testOnly 
org.apache.spark.sql.catalyst.analysis.TypeCoercionSuite'
   build/sbt 'sql/testOnly org.apache.spark.sql.DateFunctionsSuite -- -z "date 
arithmetic" -z "function date_add" -z "function date_sub"'
   ```
   
   The commands passed 63 Catalyst tests and five SQL tests. Catalyst main/test 
and
   SQL test Scalastyle checks also passed, along with `git diff --check`.
   
   ### Was this patch authored or co-authored using generative AI tooling?
   
   Generated-by: OpenAI Codex (GPT-6)
   


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