swiftlang/swift

Bad diagnostic when inheriting from class in extension

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#64,903 opened on Apr 4, 2023

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bugcompilerdiagnostics qualityextensiongood first issueinheritanceswift 6.0type checker

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Description

The following errors are misleading and could use a rewording as clear language rules, depending on the kinds of extended and inherited types.

protocol P {}
struct S {}
class C1 {}
class C2 {}

extension C2: S {} // error: inheritance from non-protocol type 'S'
extension C2: C1 {} // error: inheritance from non-protocol type 'C1'
extension C2: C1 & P {} // error: inheritance from class-constrained protocol composition type 'C1 & P'

Environment

  • Swift version 5.9-dev (LLVM 6a04c4848990c70, Swift c1d5118c21d)

NB: How to improve on these is left as an exercise to whoever takes the issue.

Contributor guide

Research direction
Locate the diagnostic emission in the Swift compiler's type checking phase (e.g., in lib/Sema/) and modify the error message strings to provide clearer explanations for non protocol and class constrained composition types.
Tech stack
swift
Domain
backend
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
SwiftCompiler internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60