relates to:

• SR-13888 Dictionary iteration by keys forces copy-on-write for struct elements

Issue Description:

If I'm iterating over a dictionary's indices and mutating the dictionary in the loop, it does a copy-on-write on the first mutation. This is a surprising and hard to discover. I verified this by printing out the variant's native instance address in LLDB to confirm reallocation.

var dict = ["one": 1, "two": 2]
for idx in dict.indices {
    dict.values[idx] += 1
}

Comparing the dict._variant.object.nativeInstance before and after the loop shows different addresses.

This is a consequence of the fact that Dictionary uses DefaultIndices, and DefaultIndices keeps a reference to the collection.

More generally, DefaultIndices does not seem to be safe to use with any mutable collection as there is no way for it to walk the collection's indices without keeping a strong reference to the collection.