Throwing method cannot be an implementation of an @objc requirement because it returns a value of type `Bool`

I have IKabaSDK.h which is a (Objective-C) protocol :

@import Foundation;
@import MobileSdk;
    
NS_ASSUME_NONNULL_BEGIN
    
@protocol IKabaSDK <NSObject>
     
- (BOOL)isStarted:(NSError* _Nullable __autoreleasing * _Nullable)error
__attribute__((swift_error(nonnull_error)))
NS_SWIFT_NAME(isStarted());

I have a (Swift) class KabaSDKThunk where the said protocol is implemented:

class KabaSDKThunk: NSObject, IKabaSDK {
    func isStarted() throws -> Bool {
        do {
            try sdk.isStarted()
            print("thunk getIsStarted")
        } catch {
            print("thunk getIsStarted throws \(error)")
            throw error
        }
    }
    ...
}

Xcode gives me these two errors next to my implementation:

• Non-@objc method isStarted() does not satisfy requirement of @objc protocol IKabaSDK
• Throwing method cannot be an implementation of an @objc requirement because it returns a value of type Bool; return Void or a type that bridges to an Objective-C class

And If I don't add Bool it complains that KabaSDKThunk does not conform to protocol IKabaSDK and suggests to add the Bool return type, so the errors are mutually exclusive. How do I solve the issue and why do we need Bool and throw here?

why do we need Bool and throw

This question boils down to what you in fact want to achieve. I agree the given behaviour is inconsistent, and moreover, the Swift bridging rules were changed in the past and is likely a subject to change in future, so what you have currently broken, can turn into working sample in foreseeable future. However for now, this error says it all:

Throwing method cannot be an implementation of an @objc requirement because it returns a value of type Bool; return Void or a type that bridges to an Objective-C class

In other words, if your Swift method is marked with throws keyword (and exposed to Objective-C runtime), it has to return a plain Objective-C type (not magically bridged Swift structures, like Int, Bool, Double, etc.. it has to be something subclassed from NSObject, NSProxy or other Objective-C entities) OR return nothing. Period. Take it as a rule (at least for now). (P.S. This specific situation indeed looks like an LLVM bug, because the same set of requirements works perfectly fine when applied to a non-protocol Objective-C method (submitted it here, so the community has a chance to review it))

Having that said, the proper workaround to this situation depends on your final goal.

"Conventional" failable method

By "conventional" here I mean a contract that Cocoa/Cocoa touch programmer would usually expect. In this scenario a method with the following signature:

- (BOOL)failableMethodWithError:(NSError **)error;

Is commonly meant to fail wherever it returns NO value (in the world of Objective-C). It's uncommon for Swift to deal with indirect parameters like NSError ** so in order to keep it consistent such methods are bridged as follows:

func failableMethod() throws

You can read more about this convention in the About Imported Cocoa Error Parameters documentation.

Failable method with preserved return type with swift_error(nonnull_error) attribute

If you want to preserve return type AND the failable signature, you have two options. First is by giving the method swift_error(nonnull_error) attribute. However in this case, in order to comply with existing bridging rules, your method has to have a type that "bridges" to an Objective-C class, e.g. NSNumber *:

- (NSNumber *)failableMethodWithError:(NSError ** _Nullable)error __attribute__((swift_error(nonnull_error)));

And here is how you implement such a method in Swift:

func failableMethod() throws -> NSNumber {
    return NSNumber(booleanLiteral: false)
}

Failable method with preserved return type with swift_error(none) attribute

Another option to preserve the return type is by disabling Objective-C - Swift error signature conversion at all with use of swift_error(none) attribute:

- (BOOL)failableMethodWithError:(NSError ** _Nullable)error __attribute__((swift_error(none)));

In this scenario you can return a scalar type from the failable method, but at the same time you will have to deal with NSErrorPointer type:

func failableMethodWithError(_ error: NSErrorPointer) -> Bool {
    if (/*error condition */) {
        error?.pointee = NSError(domain: TDWErrorDomain, code: TDWErrorDomainErrorCode)
    }
    return true
}