gistfile1.md

Builds and runs no problem:

@interface Foo : NSObject
@property (readonly) id foo;
@end

@implementation Foo

- (instancetype)init
{
  self = [super init];
	if (self)
	{
		_foo = [NSObject new];
	}
	return self;
}

@end

int main(int argc, const char * argv[])
{
	@autoreleasepool {
	    Foo *foo = [Foo new];
		NSLog(@"%@", foo.foo);
	}
    return 0;
}

Compiler error:

@interface Foo : NSObject
@property (readonly) id foo;
@end

@implementation Foo

- (instancetype)init
{
  self = [super init];
	if (self)
	{
		_foo = [NSObject new];
	}
	return self;
}

- (id)foo
{
	return _foo;
}

@end

int main(int argc, const char * argv[])
{
	@autoreleasepool {
	    Foo *foo = [Foo new];
		NSLog(@"%@", foo.foo);
	}
    return 0;
}

By implementing the sole accessor on a readonly property, you are assumed to be handling whatever synthesis or indirection you'd want for that property, so autosynthesize is disabled for that property.

To get _foo, add "@synthesize foo = _foo;"

My point was that providing _foo in some methods and not others is not a sensible or intuitive implementation.