[Rd] mean(x) != mean(rev(x)) different with x <- c(NA, NaN) for some builds

[Henrik Bengtsson]

In R 3.3.3, I observe the following on Ubuntu 16.04 (when building
from source as well as for the sudo apt r-base build):

> x <- c(NA, NaN)
> mean(x)
[1] NA
> mean(rev(x))
[1] NaN

> rowMeans(matrix(x, nrow = 1, ncol = 2))
[1] NA
> rowMeans(matrix(rev(x), nrow = 1, ncol = 2))
[1] NaN

> .rowMeans(x, m = 1, n = 2)
[1] NA
> .rowMeans(rev(x), m = 1, n = 2)
[1] NaN

> .rowSums(x, m = 1, n = 2)
[1] NA
> .rowSums(rev(x), m = 1, n = 2)
[1] NaN

> rowSums(matrix(x, nrow = 1, ncol = 2))
[1] NA
> rowSums(matrix(rev(x), nrow = 1, ncol = 2))
[1] NaN

I'd expect NA to trump NaN in all cases (with na.rm = FALSE).  sum()
does not have this problem and returns NA in both cases (*).

For the same R version build from source on RHEL 6.6 system
(completely different architecture), I get the expected result (= NA)
for all of the above cases, e.g.

> x <- c(NA, NaN)
> mean(x)
[1] NA
> mean(rev(x))
[1] NA
[...]

Before going insane trying to troubleshoot this, I have a vague memory
that this, or something related to this, has been discussed
previously, but I cannot locate it.

Is the above a bug in R, a FAQ, a build error, overzealous compiler
optimization, and / or ...?

Thanks,

Henrik