Index Checker unable to infer length properties of CharSequence length() method

Consider the following code:-

class ex{
	public static void pr(CharSequence ch) {
		char[] newCh = new char[ch.length()];
		for (int i = 0; i < newCh.length; i++)
			newCh[i] = ch.charAt(i);
	}
}

Command:-

javac -processor index ex.java

Output:-

ex.java:7: error: [argument.type.incompatible] incompatible types in argument.
        newCh[i] = ch.charAt(i);
                             ^
  found   : @LTLengthOf(value="newCh", offset="0") int
  required: @LTLengthOf(value="ch", offset={}) int
1 error

If I change code to:-

class ex{
	public static void pr(CharSequence ch) {
		String b=ch.toString();
		char[] newCh = new char[b.length()];
		for (int i = 0; i < newCh.length; i++)
			newCh[i] = ch.charAt(i);
	}
}

then it works fine.

Start by reproducing the example with javac -processor index and compare it with the ch.toString() variant. Investigate how the Index Checker handles CharSequence.length() and CharSequence.charAt() in this loop. Done means the original example type-checks without the incompatible-length error.