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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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

Research direction
Find the Swift compiler code that handles type casting diagnostics, specifically the part that checks for casts to protocol types like Equatable and Hashable. Add a diagnostic suggestion to use `AnyHashable` when such a cast is attempted.
Tech stack
swift
Domain
compilers
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.
Fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
Swift
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
80