Ignoring return value of function declared with "warn_unused_result" attribute

It should work to cast the expression to a void expression.

(void)[self.emailField becomeFirstResponder]; 

(Even I cannot reproduce the warning. However, this might depend on the warning flags.)

To your edit:

BOOL ignore = [self.emailField becomeFirstResponder]; 

Likely

BOOL ignore = [self.emailField becomeFirstResponder]; 
ignore = ignore;
// or
(void)ignore;

should remove the warning. (It does with my flags.) However, this an ugly hack, too.

BTW: There is a reason for the attribute. Maybe you should recheck, whether it is a good idea, not to test for the return value.

You could get this warning if you have a Swift function that you call from Objective-C code. You can silence it by adding @discardableResult before the function in Swift, like this:

@objc @discardableResult func yourFanzyFuncInSwift(argument: String) -> Bool { ... }

Seems like warn_unused_result is the default setting in Swift, meaning that you shouldn't write code that ignores return values.