swiftlang/swift

Bad diagnostic about memberwise `init` not being public

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#78,362 opened on Dec 24, 2024

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 (21 comments) (0 reactions) (1 assignee)Swift (69,989 stars) (10,719 forks)batch import
access controlcompilerconformancesdiagnostics qualityfix-itsgood first issuememberwise initswift 6.2type checker

Description

Description

No response

Reproduction

public struct Number: RawRepresentable {
  public let rawValue: Int
}

398 | public struct Number: RawRepresentable {
    |               `- error: initializer 'init(rawValue:)' must be declared public because it matches a requirement in public protocol 'RawRepresentable' [witness_not_accessible_proto]
399 |   public let rawValue: Int
400 | }

Expected behavior

This diagnostic is not great:

  1. What initializer? (The implicit memberwise initializer).
  2. How do I or can I at all declare it public? (An implicit memberwise initializer cannot be public).
  3. What should I do to make this work? (Declare an explicit initializer + fix-it).

Environment

Swift version 6.2-dev (LLVM 1a588e5b044c46f, Swift 55189bae8e55169)

Additional information

No response

Contributor guide

Tech stack
swift
Domain
toolingdeveloper 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
Swift language basicsUnderstanding of compiler diagnostics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
Investigate the diagnostic emission for witness not accessible proto in the Swift compiler, likely in lib/Sema or lib/AST. The error occurs when a public struct conforming to RawRepresentable has an implicit memberwise initializer. The fix should improve the diagnostic to mention the implicit memberwise init and suggest declaring an explicit public init, and potentially provide a fix it.