JuliaLang/julia

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

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#57,855 创建于 2025年3月21日

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描述

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.

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backenddeveloper experience
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bug
难度面向新贡献者的预计实现难度，1 表示很小改动，5 表示专家级工作。
3
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clear
前置要求
Julia programming basicsUnderstanding of type promotion in JuliaFamiliarity with div and divrem functions
新手友好度1-100 的估计分数，表示该议题对首次贡献者的友好程度。
25
研究方向
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.