An element write of a `T | undefined` binding holds a computed index in a
temporary ahead of its two stores, which then read the receiver after the
index. When the index can rebind the receiver (`a[(a = b, 0)] = u`), the
write now keeps reading the receiver first, so it stores into the array
the receiver held before the index ran.
The key conversion of a union index (a sequence that declares a temporary
and may read the constant "undefined" key) no longer counts as running
other code, so such an index is still held.
Chris Tate
98006639b7