Is this an undefined behaviour in Objective-C?

UB.h

#import <Foundation/Foundation.h>


@interface Foo : NSObject
@end


@interface Foo (UndefinedBehaviour)
  
- (void)foo:(id)foo bar:(id)bar;

@end


@interface Bar: Foo
@end

UB.m

#import "Foo.h"


@implementation Foo
@end

  
@implementation Foo (UndefinedBehaviour)

- (void)foo:(id)foo bar:(id)bar
{
    NSLog(@"@implementation of Foo (UndefinedBehaviour)");
}

@end

  
@implementation Bar
  
- (void)foo:(id)foo bar:(id)bar
{
    NSLog(@"@implementation Bar");
}

@end