is.numeric (NA + NA) is TRUE, should be FALSE (PR#8745)

So in
NA+NA
the logical is automatically coerced to a numerical value, and
is.numericreturns TRUE, as expected.

But for some reason, all arithmetic operations on NA's are

in fact numeric, even if it's with other NA's.

All arithmetic operatiosn on *logicals* are coerced to numeric, so if you 
have two logical NAs and add them you get a numeric NA.

This isn't anything special about NAs, since you would get the same 
conversion for TRUE+TRUE, and you would not get the same conversion for 
two character NAs

 	-thomas

Thomas Lumley			Assoc. Professor, Biostatistics
tlumley at u.washington.edu	University of Washington, Seattle