What's an "ignorable character in a Java identifier"

Question

I stumbled across this doc and wondered what that was all about. Apparently you can have certain control characters inside identifiers and they are ignored:

public static void main(String[] args) throws Exception {
    int dummy = 123;
    System.out.println(d​ummy); // Has U+200B after the `d` before the `u`
}

I couldn't find anything about this in the JLS. IntelliJ IDEA gives an error in the editor saying "dummy" is an undeclared identifier (but nevertheless it compiles and runs). I guess that's an error in IntelliJ? What purpose do these "ignoreable characters" serve?

(Note: StackOverflow seems to remove my control characters from the question)

Answer 1

There is an open issue for this contradiction.

In summary, these characters are indeed ignored for identifier name matching by the compiler but JLS doesn't mention this. Instead JLS says:

Two identifiers are the same only if they are identical, that is, have the same Unicode character for each letter or digit.

Also

A "Java letter-or-digit" is a character for which the method Character.isJavaIdentifierPart(int) returns true

The contradiction is obvious as:

Character.isJavaIdentifierPart('\u0001')  -> true, so used to compare identifier names
Character.isIdentifierIgnorable('\u0001') -> true, should be ignored actually

I speculate that Intellij IDEA follows the JLS or they are simply unaware of ignorable characters. I don't see a bug report for this here.

As to what is the purpose of these ignorables, unicode specifies some Layout and Format Control Characters. It is suggested that these characters should be ignored in identifier names as

the effects they represent are stylistic or otherwise out of scope for identifiers, and second because the characters themselves often have no visible display

Apparently the purpose of isIdentifierIgnorable is to identify characters of this category. For instance it's mentioned in the isIdentifierIgnorable documentation that it returns true for characters that have the FORMAT general category value which are characters with unicode General_Category value of Cf which are included in the Layout and Format Control Characters