Add support for <int64> * <difftime>

[MichaelChirico]

as.integer64(1) * difftime(Sys.time(), Sys.time())
# integer64
# [1] <NA>
# attr(,"units")
# [1] secs
# Warning message:
# Incompatible methods ("*.integer64", "*.difftime") for "*" 

It works fine for plain integer:

1L * difftime(Sys.time(), Sys.time())
# Time difference of -3.767014e-05 secs

[hcirellu]

As long as this is based on S3 class, this cannot be changed/fixed. The reason is the dispatch algorithm for Ops generics in R that falls back to the internal method (e.g. *) if the parameters e1 and e2 have different class-attributes.
A difftime object has class-attribute difftime and integer64 integer64. Since they are different the * is dispatched to the internal one with the above warning message.
An integer has no class-attribute. Hence the dispatch algorithm chooses *.integer64 for 1L*as.integer64(1) and as.integer64(1)*1L.
Is it worth switiching to S4?