Make Xcode 4.3 warn about undeclared methods that exist in the current @implementation

Xcode 4.3 doesn't warn about undeclared methods when they exist in the current @implementation, which is a great new feature. However, this is causing an issue in certain circumstances when using my project on Xcode 4.2.

How do I re-enable the warnings for undeclared methods?

For example:

@interface MashTun : NSObject
- (void)foo;
@end

@implementation MashTun
- (void)foo {
    CGRect rect = [self smallRect];
    NSLog(@"My Small Rect: %@", NSStringFromCGRect(rect));
}

- (CGRect)smallRect {
    return CGRectMake(0, 0, 100, 100);
}
@end

In Xcode 4.2, this fails:

warning: instance method '-smallRect' not found (return type defaults to 'id')
error: initializing 'CGRect' (aka 'struct CGRect') with an expression of incompatible type 'id' 

I completely understand the warning and error in Xcode 4.2 since it's not allowing the search for methods within the current @implementation scope. (The fix is simple: either put the smallRect method above the foo method, or declare the smallRect method in a category or the header. )

But how do I turn on a warning in Xcode 4.3 to catch this error before passing it on to colleagues running 4.2?