Because this is still haunting our customers: https://twitter.com/steipete/status/433355491436011522

PSPDFCheckAgainstIncompatible3rdPartyCode.m

BOOL PSPDFSystemFunctionOverriddenInCategory(Class theClass, SEL selector, NSString *__autoreleasing* categoryName) {
    NSCParameterAssert(theClass);
    NSCParameterAssert(selector);

    Dl_info info;
    if (dladdr(class_getMethodImplementation(theClass, selector), &info)) {
        // /System/Library/Frameworks is a common denominator, some methods are in /usr/lib/libobjc.A.dylib
        // Custom app is in /private/var/mobile/Containers/Bundle/Application/<UID>/PSPDFCatalog.app/PSPDFCatalog
        if (!strstr(info.dli_fname, "/System/Library") && !strstr(info.dli_fname, "/usr/lib")) {
            if (categoryName) *categoryName = @(info.dli_sname);
            return YES;
        }
    }
    return NO;
}

void PSPDFCheckAgainstIncompatible3rdPartyCode(void) {
    // Is - [NSArray firstObject] overridden?
    NSString *categoryName;
    if (PSPDFSystemFunctionOverriddenInCategory(NSArray.class, @selector(firstObject), &categoryName)) {
        // Test our assumption of the faulty category.
        @try { [@[] firstObject]; }
        @catch (__unused NSException *exception) {
            PSPDFLogError(@"The method `firstObject` of NSArray is overridden in the category %@, which performs differently than Apple's default. Check your categories and 3rd-party code and remove/update the culprit. We're repairing that for now.", categoryName);
            // Repair it.
            PSPDFSwizzleMethodImplementation(NSArray.class, @selector(firstObject), ^id(NSArray *_self) {
                return _self.count ? _self[0] : nil;
            });
        }
    }
}