swiftlang/swift

Offer additional note for why method with Self reference cannot be used on `any SomeType`

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#76,320 opened on Sep 6, 2024

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compilerdiagnostics qualityexistentialsgood first issueimprovementswift 6.2type checker

Description

Motivation

The following snippet;

protocol Base {
    associatedtype SomeID: Hashable
}

protocol Alpha: Base {
    func take(id: Self.SomeID)
}

struct Use {
    let alpha: any Alpha
    
    func test() {
        alpha.take(id: 12) // error: Member 'take' cannot be used on value of type 'any Alpha'; consider using a generic constraint instead
    }
}

is correct in rejecting the line, since we need Self.SomeID to be well typed, but we dont know what its type is via the any.

It'd be helpful if we can point out that "why" the method can't be used via a note, in a method with many parameters one can sometimes miss the Self reference.

Proposed solution

Adding a note on the offending parameter would be good

Alternatives considered

No response

Additional information

No response

Contributor guide

Tech stack
swift
Domain
developer experiencetooling
Issue type
feature
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 genericscompiler diagnosticsexistential types
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
55
Research direction
Investigate the error diagnostic emitter for type mismatch involving Self associated types. Look at the error message generation for 'cannot use member' and add a note highlighting the Self.SomeID parameter. The relevant code is likely in the Sema or Diagnostics directory. Review comments from maintainers in the issue for guidance.