JuliaLang/julia

Document promotion behavior of `div` and `divrem` for `(::Unsigned, ::Signed)`

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#57855 opened on Mar 21, 2025

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docshelp wanted

Description

Currently we have

julia> div(UInt(1), -1)
0xffffffffffffffff

This happens because we are returning the promote_type, but violates both common sense, and the documentation which states "Generally equivalent to a mathematical operation x/y without a fractional part".

As such, I propose that we instead make the return type of div(x,y) be ifelse(isa(y, Signed), signed(promote_type(x,y)), promote_type(x,y)). This would obviously be slightly breaking, but I think it qualifies as a bugfix.

We also would want to make similar changes for divrem, fld, cld since they all have the same problem.

Contributor guide

Tech stack
None
Domain
backenddeveloper experience
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
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
Julia programming basicsUnderstanding of type promotion in JuliaFamiliarity with div and divrem functions
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25
Research direction
Investigate the implementation of `div` and `divrem` in Julia's base library, likely in `intfuncs.jl`. Understand the current type promotion behavior for mixed `(Unsigned, Signed)` arguments. Implement the proposed change to return a signed type when the divisor is signed. Consider similar changes for `fld` and `cld`. Check for existing tests and add new ones to cover the corrected behavior. Review the discussion in the issue for any additional requirements or concerns.