How to get rid of the 'undeclared selector' warning

Have a look at NSSelectorFromString.

 SEL selector = NSSelectorFromString(@"setError:");
 if ([self respondsToSelector:selector])

It will allow you to create a selector at runtime, instead of at compile time through the @selector keyword, and the compiler will have no chance to complain.

Another option would be to disable the warning with:

#pragma GCC diagnostic ignored "-Wundeclared-selector"

You can place this line in the .m file where the warning occurs.

Update:

It works also with LLVM like this:

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wundeclared-selector"

... your code here ...

#pragma clang diagnostic pop