typelevel/cats

Make `Hash.fromUniversalHashCode` null safe

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#4831 opened on Feb 22, 2026

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 (6 comments) (1 reaction) (1 assignee)Scala (5,438 stars) (1,241 forks)batch import
good first issue

Description

Similarly to how Show.fromToString have been recently made null safe (#4689 and #4790), I propose making Hash.fromUniversalHashCode also null safe. Specifically, I think it should use x.## instead of x.hashCode(). (The eqv method is already fine, as it uses ==.)

Contributor guide

Tech stack
scala
Domain
backend
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
ScalaCats
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70
Research direction
Review the approach used in PRs #4689 and #4790 to make `Show.fromToString` null safe. Locate `Hash.fromUniversalHashCode` in the Cats source code and apply the same pattern: replace `x.hashCode()` with `x.##`. Verify the `eqv` method already uses `==` and does not need changes. Check for existing tests that cover this method and update them accordingly.