protocol ParentError: Error {}

struct ChildError1: ParentError {}
struct ChildError2: ParentError {}

func foo<SomeError: ParentError>(_ mockResult: Result<String, ChildError1>, _: Result<String, SomeError>) {
    if let castedResult = mockResult as? Result<String, SomeError> {
        print("The warning is wrong: \(castedResult)")
    } else {
        fatalError("If the warning is true it should crash here")
    }
}

foo(.success("mock"), Result<String, ChildError1>.success("ignored"))
// foo(.success("mock"), Result<String, ChildError2>.success("ignored")) // This case does crash

foo.swift:7:38: warning: cast from 'Result<String, ChildError1>' to unrelated type 'Result<String, SomeError>' always fails
    if let castedResult = mockResult as? Result<String, SomeError> {
                          ~~~~~~~~~~ ^   ~~~~~~~~~~~~~~~~~~~~~~~~~
The warning is wrong: success("mock")