FULL PRODUCT VERSION :
Java 9: OpenJDK Runtime Environment (build 9.0.1+11)
Java 10: Java(TM) SE Runtime Environment (build 10-ea+30)

A DESCRIPTION OF THE PROBLEM :
A ClassCastException is thrown from the internals of DateTimeFormatterBuilder when building a format using padding, e.g. DateTimeFormatter.ofPattern("pdQ");

The exception message is: java.base/java.time.format.DateTimeFormatterBuilder$PadPrinterParserDecorator cannot be cast to java.base/java.time.format.DateTimeFormatterBuilder$NumberPrinterParser

This bug was found using AFL + JQF.


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Compile and run the attached main class DateTimeFormatterTest


ERROR MESSAGES/STACK TRACES THAT OCCUR :
Exception in thread "main" java.lang.ClassCastException: java.base/java.time.format.DateTimeFormatterBuilder$PadPrinterParserDecorator cannot be cast to java.base/java.time.format.DateTimeFormatterBuilder$NumberPrinterParser
at java.base/java.time.format.DateTimeFormatterBuilder.appendValue(DateTimeFormatterBuilder.java:636)
at java.base/java.time.format.DateTimeFormatterBuilder.appendValue(DateTimeFormatterBuilder.java:394)
at java.base/java.time.format.DateTimeFormatterBuilder.parseField(DateTimeFormatterBuilder.java:1876)
at java.base/java.time.format.DateTimeFormatterBuilder.parsePattern(DateTimeFormatterBuilder.java:1723)
at java.base/java.time.format.DateTimeFormatterBuilder.appendPattern(DateTimeFormatterBuilder.java:1691)
at java.base/java.time.format.DateTimeFormatter.ofPattern(DateTimeFormatter.java:559)
at DateTimeFormatterTest.main(DateTimeFormatterTest.java:5)


REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.time.format.DateTimeFormatter;

public class DateTimeFormatterTest {
    public static void main(String args[]) {
        DateTimeFormatter.ofPattern("pdQ");
    }
}

---------- END SOURCE ----------