[SR-12459] Cannot call equality operator declared as a static member of a protocol

[ahoppen]

Previous ID	SR-12459
Radar	rdar://problem/62202443
Original Reporter	@ahoppen
Type	Bug
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, DiagnosticsQoI, TypeChecker
Assignee	@ahoppen
Priority	Medium

md5: 886bc3d5b61b666595f45872e20f5f63

Issue Description:

Take a look at the following code:

protocol Foo {}


extension Foo {
  static func ==(lhs: Foo, rhs: Foo) -> Bool {
    fatalError()
  }
}


struct Bar: Foo {}

let a: Foo = Bar()
let b: Foo = Bar()
print(a == b) // Error: Binary operator '==' cannot be applied to two 'Foo' operands

I would expect a == b to call the == operator declared in the extension of Foo. However, I get an error „Binary operator '==' cannot be applied to two 'Foo' operands“.

Using Swift 5.2 shipped with Xcode 11.4 (11E146, swiftlang-1103.0.32.1)

[hamishknight]

The problem is that there's no way to infer the type for the implicit Self parameter, we should probably emit a warning explaining this to the user and suggest making it a top-level operator function instead.

[ahoppen]

A warning sounds like a good solution to me if it's not easy to get it to work 👍

[beccadax]

@swift-ci create

[theblixguy]

On master, the diagnostic now says:

/Users/spare/Desktop/test.swift:15:9: error: generic parameter 'Self' could not be inferred
print(a == b)
        ^

[xedin]

I think diagnostic about `generic parameter 'Self' could not be inferred` makes sense here.