Mishandling missing "..." (PR#1247)

R> myfun <- function(x, ...) {x[...] <- 0; x}
R> myfun(3)
   Error in myfun(3) : SubAssignArgs: invalid number of arguments

It fails because no ... was passed.  The workaround (and desired behavior) is:
R> myfun <- function(x, ...) {if (missing(...)) x[] <- 0 else x[...] <- 0; x}
R> myfun(3)
   [1] 0

This is interesting.  Another workaround is

myfun

function(x, ...) {x[...] <- 0; x}

myfun(3,)

[1] 0

and the problem goes down to eval.c:evalListKeepMissing, which says
	/* If we have a ... symbol, we look to see what it is bound to.
	 * If its binding is Null (i.e. zero length)
	 *	we just ignore it and return the cdr with all its
               expressions evaluated;

The difference between  myfun(3) and myfun(3,) is that in the former case
there are no ... arguments, in the latter there is a ... argument but it's
missing.

In one way this would be fairly easy to fix by writing yet another variant
of evalList that replaced an empty ... with R_MissingArg, but I'm scared.


	-thomas

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