Try/Catch DU Exception

[Archie Cobbs]

I'd like to propose a very minor language improvement and would appreciate
any feedback.

This is a true corner case, but I bet most developers have tripped over it
a few times. It's easy to work around... but still...

Here's a simple example:

    void describeMyThread(Thread thread) {
        final String description;
        try {
            description = '"' + thread.getName() + "'";
        } catch (NullPointerException e) {
            description = "null";
        }
        System.out.println("the thread is " + description);
    }

This doesn't compile:

    DUTest.java:8: error: variable description might already have been
assigned
                description = "(null)";
                ^

The error is a false positive: there is no way an exception can be thrown
in the try block *after* description is assigned, because description being
assigned is literally the last thing that occurs in the try block.

Developers intuitively know that description will be DU at the start of the
catch block, so the error feels surprising and makes the compiler seem less
smart than it should be.

My proposal is to fix this by adding a "try/catch DU exception" to §16.2.15:

V is definitely unassigned before a catch block iff all of the following
are true:
    V is definitely unassigned after the try block *or the try block ends
with an assignment expression statement V=b and V is definitely unassigned
after b*
    V is definitely unassigned before every return statement ...

A prototype compiler implementation is here
<https://github.com/openjdk/jdk/compare/master...archiecobbs:jdk:TryCatchDUException>
.

Thoughts?

-Archie

-- 
Archie L. Cobbs
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