[Issue 20476] New: chainTogether leaks exception with -dip1008

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20476

          Issue ID: 20476
           Summary: chainTogether leaks exception with -dip1008
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: druntime
          Assignee: nobody at puremagic.com
          Reporter: moonlightsentinel at disroot.org

Test case:

class E : Exception
{
    static int instances;
    this() {
        super("MSG");
        instances++;
    }

    ~this() {
        instances--;
    }
}

void main()
{
    try
    {
        throw new E();
    }
    catch (Throwable e)
    {
        assert(E.instances == 1);
    }

    assert(E.instances == 0);

    try
    {
        throw new E();
    }
    catch (Throwable e)
    {
        assert(E.instances == 1);
        e = Exception.chainTogether(null, e); // Returns e unchanged
    }

    assert(E.instances == 0);
}

The last assert fails because chainTogether returns e directly (due to the
first throwable being null) but increments the refcount anyway

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