swiftlang/swift

[SR-14044] Contextual Usage of UnsafeMutablePointer<Void> In API is Mis-Diagnosed

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#56,435 opened on Jan 13, 2021

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compilergood first issueimprovementtype checker

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Description
Previous ID SR-14044
Radar rdar://43580528
Original Reporter @CodaFi
Type Improvement
Votes 0
Component/s Compiler
Labels Improvement, StarterBug, TypeChecker
Assignee None
Priority Medium

md5: 29439788662577468de9cf1437c1e59a

Issue Description:

import Foundation

var data = Data(count:0)
try data.withUnsafeMutableBytes { (ptr: UnsafeMutablePointer<Void>) throws -> Void in
}

There is a warning here because TypeCheckType only knows how to unconditionally warn about UnsafeMutablePointer<Void>. It needs to be taught that this contextual usage is okay - because the alternative is the compiler emitting a fixit that introduces a contextual mismatch by rewriting the parameter type to UnsafeMutableRawPointer.

Contributor guide

Research direction
Locate the type checker code that emits the warning for UnsafeMutablePointer<Void> and modify it to skip the warning when the pointer type is used contextually (e.g., as a parameter type in a closure passed to withUnsafeMutableBytes).
Tech stack
swift
Domain
tooling
Issue type
Bug
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.
Stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
SwiftGit
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
80