Currently, we force the temporary symbol for an index expression in a self-assignment to get a slot. It used to be needed to avoid double evaluation of the index, e.g. in

x[y()] += z

However, these days we compile this to:

    ALOAD 1
    DUP
    ALOAD 2
    ACONST_NULL
    INVOKEDYNAMIC dyn:call
    ASTORE 5
    ALOAD 5
    DUP_X1
    INVOKEDYNAMIC dyn:getElem
    ALOAD 3
    INVOKEDYNAMIC ADD:OOO_I
    INVOKEDYNAMIC dyn:setElem

Note that due to the use of DUP_X1, the ASTORE 5 and ALOAD 5 are not necessary. If we didn't use DUP_X1, then indeed we'd need the local variable slot:

    ASTORE 5
    ALOAD 5
    INVOKEDYNAMIC dyn:getElem
    ALOAD 3
    INVOKEDYNAMIC ADD:OOO_I
-> ALOAD 5
    INVOKEDYNAMIC dyn:setElem

It should be investigated if we can eliminate the ASTORE/ALOAD in every self-assignment scenario.