ADDITIONAL SYSTEM INFORMATION :
Windows 10 64-bits.

Reproducible with all of:

* openjdk version "1.8.0_275"
* openjdk version "11.0.9" 2020-10-20
* openjdk version "15.0.1" 2020-10-20
* openjdk version "17-ea" 2021-09-14

A DESCRIPTION OF THE PROBLEM :
When using flags other than `-` with the `%%` conversion (for example with the string "%+%"), the JavaDoc at
https://docs.oracle.com/en/java/javase/15/docs/api/java.base/java/util/Formatter.html#dper
specifies that a FormatFlagsConversionMismatchException should be thrown. However, an IllegalFormatFlagsException is thrown instead.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Using the provided test file below:

$ javac -d bin test/Test.java
$ java -cp bin test.Test


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
No exception (each invocation should throw the exception that is specifically caught in the surrounding try/catch).
ACTUAL -
Exception in thread "main" java.util.IllegalFormatFlagsException: Flags = '+'
        at java.base/java.util.Formatter$FormatSpecifier.checkText(Formatter.java:3210)
        at java.base/java.util.Formatter$FormatSpecifier.<init>(Formatter.java:2897)
        at java.base/java.util.Formatter.parse(Formatter.java:2729)
        at java.base/java.util.Formatter.format(Formatter.java:2671)
        at java.base/java.util.Formatter.format(Formatter.java:2625)
        at java.base/java.lang.String.format(String.java:3290)
        at test.Test.main(Test.java:26)

(line 26 is the last call to String.format in the reproducing code)

---------- BEGIN SOURCE ----------
package test;

import java.util.*;

public class Test {
  public static void main(String[] args) {
    try {
      // throws IllegalFormatFlagsException, as expected
      String.format("%+ %");
    } catch (IllegalFormatFlagsException e) {
      // ok
    }

    try {
      // throws IllegalFormatFlagsException, as expected as well
      // because %n has a special spec
      String.format("%+n");
    } catch (IllegalFormatFlagsException e) {
      // ok
    }

    try {
      // throws IllegalFormatFlagsException *as well*, although
      // the documentation says it should be
      // FormatFlagsConversionMismatchException
      String.format("%+%");
    } catch (FormatFlagsConversionMismatchException e) {
      // ok (unreachable)
    }
  }
}

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

CUSTOMER SUBMITTED WORKAROUND :
Do not use incorrect format specifiers in `format`.

FREQUENCY : always