NSObject conformance to Equatable is stealthy

[igor-makarov]

Description

NSObject has a conformance to Equatable which delegates to -[isEqual:].

On the face of it, this is not a bad idea: use Objective-C's conventions where applicable.

However, most classes do not override -[isEqual:] and use the default implementation, causing all sorts of unexpected bugs.

While it's impractical to warn users of default implementation pitfalls in each case, it might be useful to provide a diagnostic where this is possible. For example, #72765 is caused by such a default conformance.

Reproduction

class ClassA: NSObject, Equatable {} // error: Redundant conformance of 'ClassA' to protocol 'Equatable'
protocol SneakyProtocol: Equatable {}
class ClassB: NSObject, SneakyProtocol {} // no problem 

Expected behavior

Some kind of warning about the default conformance undermining the user's intent.

Environment

swift-driver version: 1.87.3 Apple Swift version 5.9.2 (swiftlang-5.9.2.2.56 clang-1500.1.0.2.5)
Target: arm64-apple-macosx14.0

Additional information

No response