-[NSArray filteredArrayUsingPredicate:] should ignore NSNull values

[triplef]

The following code runs fine on Apple platforms and filters for the second array entry, but with GNUstep it raises an exception Unable to find value for key "key" of object <null> (NSNull):

NSArray *array = @[@{@"key": @"value1"}, @{@"key": @"value2"}, NSNull.null];
NSArray *filtered = [array filteredArrayUsingPredicate:[NSPredicate predicateWithFormat:@"key == %@", @"value2"]];
NSLog(@"array: %@", array);
NSLog(@"filtered: %@", filtered);

[hmelder]

Internally, we use GSKeyPathExpression to get the key using Key-Value-Coding. When following the backtrace on lldb, you end up at the super class method of evaluateWithObject: for whatever reasons.

In this case the predicate is parsed as a NSComparisonPredicate with a GSKeyPathExpression.

TLDR:

This boils down to how we handle KVCoding with NSNull.

Given this test program:

#import <Foundation/Foundation.h>

int main(void) {
  id obj = [[NSNull null] valueForKeyPath: @"key"];
  NSLog(@"Output: %@", obj);
  return 0;
}

The following is returned on macOS: 2023-12-09 19:32:08.434 a.out[6427:263545] Output: <null>
While on Debian with GNUstep:

[hmelder]

I managed to resolve the issue by overriding the valueForUndefinedKey method in NSNull. This prevents the triggering of the default handler, which typically results in an exception.

It seems that Apple uses a similar approach in their implementation.

[hmelder]

Fixed with #358

[triplef]

Thank you for the fix!