[mikamikasuki]

Rationale for this change

Safe decimal casts to Decimal32 can silently change values or reject exact conversions because the input coefficient is narrowed before rescaling and precision checks.

What changes are included in this PR?

Keep wider input decimals at their source width during safe rescaling, then narrow to Decimal32 only after the target precision check succeeds.

Fixes #53288.

Are these changes tested?

Added regression cases for overflow, negative-scale exact rescaling, and fractional-scale exact rescaling. The regression fails with the previous conversion path and passes with this change. The full arrow-compute-scalar-cast-test executable passes (108 tests).

Are there any user-facing changes?

Safe casts now return the correct value or an overflow error. No public API changes.

This PR contains a "Critical Fix". Safe casts could previously return an incorrect decimal value or reject an exact conversion.

Was AI used for this PR?

OpenAI Codex assisted with investigation, implementation, regression-test development, and review. The new tests were run against current main (2c800e6) before and after the fix.

PR code and description written by:

• Human
• AI

Reviewed before submission by:

• Human
• AI
• Not reviewed

[github-actions]

❌ GitHub issue #53288 could not be retrieved.

[github-actions]

Thanks for opening a pull request!

This pull request has been automatically closed because you currently have 4 open pull requests, which is more than the limit of 3.

Due to the increase in pull requests opened by AI bots, and in order to keep the review queue manageable, Apache Arrow limits contributors without repository access to at most 3 concurrently open pull requests. This helps make sure each pull request gets the attention it needs and that work in progress does not go stale.

Once one of your other open pull requests has been merged or closed, you are welcome to reopen this one.

See also:

• Contribution Guidelines - Limit on concurrent pull requests
• Contribution Guidelines - Contributing Overview

[vera-rubin]

The linked issue #53288 is visible again (it was hidden for a while), so the bot's reason for closing this no longer applies, in case anyone wants to reopen it.