FULL PRODUCT VERSION :
java version "9"
Java(TM) SE Runtime Environment (build 9+181)
Java HotSpot(TM) 64-Bit Server VM (build 9+181, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [Version 10.0.10240]

EXTRA RELEVANT SYSTEM CONFIGURATION :
Running Jshell in Cmder

A DESCRIPTION OF THE PROBLEM :
Auto-completion after a /drop command resulted in StringIndexOutOfBoundsException. Here is the full output of the commands:

jshell> /vars
| Set<String> myMutableSet = [four, one, two, three, five]
| Set<String> unmodifiableSet = [four, one, two, three, five]
| boolean $8 = true

jshell> /drop myMutableSet unmoException in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: -1
        at java.base/java.lang.String.substring(String.java:1852)
        at jdk.jshell/jdk.internal.jshell.tool.ConsoleIOContext$OrdinaryCompletionTask.perform(ConsoleIOContext.java:475)
        at jdk.jshell/jdk.internal.jshell.tool.ConsoleIOContext.complete(ConsoleIOContext.java:287)
        at jdk.jshell/jdk.internal.jshell.tool.ConsoleIOContext.access$000(ConsoleIOContext.java:69)
        at jdk.jshell/jdk.internal.jshell.tool.ConsoleIOContext$1.complete(ConsoleIOContext.java:100)
        at jdk.internal.le/jdk.internal.jline.console.ConsoleReader.readLine(ConsoleReader.java:2684)
        at jdk.internal.le/jdk.internal.jline.console.ConsoleReader.readLine(ConsoleReader.java:2279)
        at jdk.jshell/jdk.internal.jshell.tool.ConsoleIOContext.readLine(ConsoleIOContext.java:139)
        at jdk.jshell/jdk.internal.jshell.tool.JShellTool.run(JShellTool.java:1140)
        at jdk.jshell/jdk.internal.jshell.tool.JShellTool.start(JShellTool.java:929)
        at jdk.jshell/jdk.internal.jshell.tool.JShellToolBuilder.run(JShellToolBuilder.java:239)
        at jdk.jshell/jdk.internal.jshell.tool.JShellToolProvider.main(JShellToolProvider.java:119)

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Start Jshell and type in the following commands:
1. String foo = "foo", bar = "bar"
2. /drop foo b<Tab>

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The "bar" variable name is auto-completed.
ACTUAL -
java.lang.StringIndexOutOfBoundsException is thrown

ERROR MESSAGES/STACK TRACES THAT OCCUR :
jshell> String foo = "foo", bar = "bar"
foo ==> "foo"
bar ==> "bar"

jshell> /drop foo
delete a source entry referenced by name or id

<press tab again to see full documentation>

jshell> /drop foo bException in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: -1
        at java.base/java.lang.String.substring(String.java:1852)
        at jdk.jshell/jdk.internal.jshell.tool.ConsoleIOContext$OrdinaryCompletionTask.perform(ConsoleIOContext.java:475)
        at jdk.jshell/jdk.internal.jshell.tool.ConsoleIOContext.complete(ConsoleIOContext.java:287)
        at jdk.jshell/jdk.internal.jshell.tool.ConsoleIOContext.access$000(ConsoleIOContext.java:69)
        at jdk.jshell/jdk.internal.jshell.tool.ConsoleIOContext$1.complete(ConsoleIOContext.java:100)
        at jdk.internal.le/jdk.internal.jline.console.ConsoleReader.readLine(ConsoleReader.java:2684)
        at jdk.internal.le/jdk.internal.jline.console.ConsoleReader.readLine(ConsoleReader.java:2279)
        at jdk.jshell/jdk.internal.jshell.tool.ConsoleIOContext.readLine(ConsoleIOContext.java:139)
        at jdk.jshell/jdk.internal.jshell.tool.JShellTool.run(JShellTool.java:1140)
        at jdk.jshell/jdk.internal.jshell.tool.JShellTool.start(JShellTool.java:929)
        at jdk.jshell/jdk.internal.jshell.tool.JShellToolBuilder.run(JShellToolBuilder.java:239)
        at jdk.jshell/jdk.internal.jshell.tool.JShellToolProvider.main(JShellToolProvider.java:119)


REPRODUCIBILITY :
This bug can be reproduced always.

CUSTOMER SUBMITTED WORKAROUND :
The workaround is to not use auto-completion when dropping multiple vars in Jshell.