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itchyny
df24981307
`reduce f as $x (init; update)` evaluated `f` against a stack slot
that forkpoints created while evaluating `init` restore. Both the
`DUPN` opening the reduction loop and the `LOADVN` retrieving the
result pop that slot destructively, nulling it out, so an `init`
that backtracks (e.g. `0, 0`) made `f` see `null` and fail with
"Cannot iterate over null".
Duplicate the input before the `FORK` instead of inside the loop,
and drop the leftover copy with `POP` once the loop is exhausted.
The slot left live at the `FORK` is then a block pushed after
`init` completed, which no earlier forkpoint can reach, so nulling
it is harmless. This is the same approach `gen_foreach` has used
since c70ae1a.
`DUPN` had no other user, so remove the opcode.