Type checking assignments.

gistfile1.m

NSDictionary *aDictionary = nil;
NSArray *anArray = nil;

void (^aBlock)() = ^{ };
// no warning
aDictionary = aBlock;
// no warning
anArray = aBlock;
// warning assigning NSArray to NSDictionary, but not for block assignment
aDictionary = anArray = aBlock;

// Similarly:
        
id anUntypedObject = nil;
// no warning
aDictionary = anUntypedObject;
// no warning
anArray = anUntypedObject;
// warning assigning NSArray to NSDictionary, but not for block assignment
aDictionary = anArray = anUntypedObject;