swiftlang/swift

[SR-5819] Suggest `as? AnyHashable` when `as? Equatable` or `as? Hashable` is attempted

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#48,389 opened on Sep 1, 2017

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 (8 comments) (0 reactions) (0 assignees)Swift (69,989 stars) (10,719 forks)batch import
bugcompilerdiagnostics qualitygood first issue

Description
Previous ID SR-5819
Radar None
Original Reporter @jckarter
Type Bug
Votes 0
Component/s Compiler
Labels Bug, DiagnosticsQoI, StarterBug
Assignee None
Priority Medium

md5: 5e4ffb6d40f711123adf86651f9a3a8f

Issue Description:

If you have two values of type Any or otherwise opaque type and you want to test whether they're equal, it's natural to try to write something like:

if let a = a as? Equatable, b = b as? Equatable {
  return a == b
}

Maybe someday that will work, but in the meantime dynamically casting to AnyHashable can get the desired effect. It'd be nice QoI to have a diagnostic telling people they can do this when they try similar things that don't.

Contributor guide

Tech stack
swift
Domain
developer experience
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.
half day
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
SwiftCompiler diagnosticsType casting
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70
Research direction
The issue requests a new diagnostic in the Swift compiler when a user attempts to cast to `Equatable` or `Hashable` on a value of opaque type. The fix should suggest `as? AnyHashable` as an alternative. To implement, examine the type checker's logic for handling dynamic casts, likely in `lib/Sema/CastCheck.cpp` or similar. Review the existing diagnostic patterns for fix its. Since this is a QoI improvement, also consider the `DiagnosticsEngine` and how to emit suggestions. The issue has comments that may discuss prior attempts or design considerations.
[SR-5819] Suggest `as? AnyHashable` when `as? Equatable` or `as? Hashable` is attempted · swiftlang/swift#48389 | Good First Issue