Stubbing a throwing async function shows compile error

[alschmut]

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Overview

Stubbing a throwing async function to throw an error shows a compile error:

givenSwift(await bird.canFly()).willThrow(MockError())
// Compiler error:
// Cannot convert value of type 'Mockable<ThrowingAsyncFunctionDeclaration, () async throws -> Bool, Bool>' 
// to expected argument type    'Mockable<ThrowingFunctionDeclaration, () async throws -> Bool, Bool>'

If I look into the mockingbird code, it seems the function willThrow(_:) is not available for the ThrowingAsyncFunctionDeclaration (see Stubbing.swift#L399)

extension StubbingManager where DeclarationType == ThrowingAsyncFunctionDeclaration {
    public func willThrow(_ error: Error) -> Self { ... }
}

Example

func test_bird_can_not_fly_after_shaking_tree() async throws {
    let bird = mock(Bird.self)
    let tree = Tree(with: bird)
    givenSwift(await bird.canFly()).willThrow(MockError()) // compile error

    try await tree.shake()

    verify(bird.fly()).wasNeverCalled()
}

protocol Bird {
    func canFly() async throws -> Bool
    func fly()
}

struct MockError: Error {}

class Tree {
    private let bird: any Bird
    
    init(with bird: any Bird) {
        self.bird = bird
    }
    
    func shake() async throws {
        if try await bird.canFly() {
            bird.fly()
        }
    }
}

Expected Behavior

When stubbing a throwing async function to throw an error
Then I expect to see no compile error
And I expect the function to throw an error, whenever it is called by the test

Environment

• Mockingbird CLI version: 0.17.0 (not sure why this is on 0.17.0, the resolved SPM package in Xcode has version 0.20.0)
• Xcode and Swift version: 5.6
• Package manager: SPM package
• Unit testing framework: XCTest
• Custom configuration
  • Mockingbird ignore files
  • Supporting source files

[ailtonvivaz]

Unfortunately, I was unable to make it work, but there is a workaround for it.
You can use:

givenSwift(await bird.canFly()).will { throw MockError() }

or even:

given(await bird.canFly()) ~> { throw MockError() }

[oleksandryevdokymov]

I have the same issue.
But a bit different workaround works for me based on @ailtonvivaz answer.
In my case error is enum that conforms Error protocol.
It looks like:

givenSwift(await ache.getContacts()).will { _ in throw error }