Generate generic mocks for protocols with associated types

[wvteijlingen-npo]

Given the following protocol:

protocol Foo<Output> {
    associatedtype Output
    func fetch() -> Output
}

The current generated mock uses Any for the associated type. This makes it awkward to use the mock, because we need to typecast it at the usage site since Any is usually not the concrete type we want. I know we can specify a concrete type using /// @mockable(typealias: Output = String), but that defeats the purpose of the associated type by limiting it to a single concrete type.

Current

class FooMock: Foo {
    init() { }

    typealias Output = Any


    private(set) var fetchCallCount = 0
    var fetchHandler: (() async throws -> Output)?
    func fetch() async throws -> Output {
        fetchCallCount += 1
        if let fetchHandler = fetchHandler {
            return try await fetchHandler()
        }
        fatalError("fetchHandler returns can't have a default value thus its handler must be set")
    }
}

Expected

Would it instead be possible to generate a mock like this?

class FooMock<T>: Foo {
    init() { }

    typealias Output = T

   // ... etc
}

[sidepelican]

The default behavior of being Any might be good to optionally disable.

Quick hack: Adding constraints to T would generate a generic mock.

protocol Foo<Output> {
    associatedtype Output: Any
    func fetch() -> Output
}

mockolo/Tests/TestPATs/FixturePAT.swift

Lines 115 to 133 in af94fc5

	/// @mockable
	public protocol Foo {
	associatedtype T: StringProtocol, Sendable
	}
	/// @mockable(typealias: T = String)
	public protocol Bar {
	associatedtype T: StringProtocol, Sendable
	}
	/// @mockable
	public protocol Baz {
	associatedtype T where T: StringProtocol, T: Sendable
	}
	@Fixture enum expected {
	public class FooMock<T: StringProtocol & Sendable>: Foo {
	public init() { }
	}