FormatString validator complains about incompatible flags even though they are compatible

[MinnDevelopment]

Example:

public class Example {
    @SuppressWarnings("FormatString")
    public static void main(String[] args){
        System.out.printf("%s\n", new ExampleFormattable());
        System.out.printf("%#s\n", new ExampleFormattable());
    }
}

class ExampleFormattable implements Formattable {
    @Override
    public void formatTo(Formatter formatter, int flags, int width, int precision) {
        if ((flags & FormattableFlags.ALTERNATE) != 0) {
            formatter.format("alternate");
        } else {
            formatter.format("normal");
        }
    }
}

Output:

normal
alternate

Compile Error without @SuppressWarnings("FormatString"):

Example.java:10: error: [FormatString] format specifier '%s' is not compatible with the given flag(s): #
        System.out.printf("%#s\n", new ExampleFormattable());
                         ^
    (see https://errorprone.info/bugpattern/FormatString)

This is likely caused by the validator using null for its check, which will throw an error.

[graememorgan]

Have you seen this come up in practice, or is this a theoretical bug? I've not seen %#s used before.

[MinnDevelopment]

We use it in JDA for various types, for instance User has %s as User (name only) and %#s as User#1234 (name + discriminator). I noticed it because our example raised an error with errorpone. It is probably not a common use-case, but I wanted to report the issue nontheless.

[graememorgan]

Fixed by #5462.