SwiftUser.swift

import UIKit

class SwiftUser: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        TestLib().myFunc()
    }

}

TestLib.swift

import Foundation

@objc class TestLib: NSObject {
    @objc fileprivate func _myFunc() {
        print("I am here")
    }
}

TestLib_Objc.h

#import "test-Swift.h"

@interface TestLib (Ext)
-(void)myFunc NS_SWIFT_UNAVAILABLE("objc only");
@end

TestLib_Objc.m

#import "TestLib_Objc.h"

@implementation TestLib(Ext)
-(void)myFunc {
    [self performSelector:NSSelectorFromString(@"_myFunc")];
}

@end


@interface TestObjcUser: NSObject
@end
@implementation TestObjcUser

- (instancetype)init
{
    self = [super init];
    if (self) {
        [[[TestLib alloc] init] myFunc];
    }
    return self;
}

@end