[Rd] Should 0L * NA_integer_ be 0L?

[Michael Chirico]

I don't see this specific case documented anywhere (I also tried to search
the r-devel archives, as well as I could); the only close reference
mentions NA & FALSE = FALSE, NA | TRUE = TRUE. And there's also this
snippet from R-lang:

In cases where the result of the operation would be the same for all
> possible values the NA could take, the operation may return this value.
>

This begs the question -- shouldn't 0L * NA_integer_ be 0L?

Because this is an integer operation, and according to this definition of
NA:

Missing values in the statistical sense, that is, variables whose value
> is not known, have the value @code{NA}
>

NA_integer_ should be an unknown integer value between -2^31+1 and 2^31-1.
Multiplying any of these values by 0 results in 0 -- that is, the result of
the operation would be 0 for all possible values the NA could take.

This came up from what seems like an inconsistency to me:

all(NA, FALSE)
# [1] FALSE
NA * FALSE
# [1] NA

I agree with all(NA, FALSE) being FALSE because we know for sure that all
cannot be true. The same can be said of the multiplication -- whether NA
represents TRUE or FALSE, the resulting value is 0 (FALSE).

I also agree with the numeric case, FWIW: NA_real_ * 0 has to be NA_real_,
because NA_real_ could be Inf or NaN, for both of which multiplication by 0
gives NaN, hence 0 * NA_real_ is either 0 or NaN, hence it must be NA_real_.

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