A filelist may include files that do not exist.  How could you copy such a file?
 If you really needed to do this, you could write a macro to touch the files
from a filelist, convert it to a fileset, and then copy them.

<copy> could fail in this situation.

Which situation?  Using filelists, or using the touch workaround?

I meant if a file does not exist, <copy> should just fail.

Then, again, why allow filelists in the first place, if copying a nonexistent
file triggers failure?

Why not think in broader terms than just <copy>, and provide a way to create a
fileset from a filelist, one that -on the point of creation- resolves what the
policy on missing files is. That is, it can skip them, or it can fail.

-steve

Creating a fileset from a filelist should already be doable using pathconvert. 
So all that is really needed is a condition asserting that all files in a
filelist exist, no?

Or, one could create the fileset from the filelist using a pathconvert as
suggested, then do another pathconvert on the created fileset and if the two are
not equal, files are missing from the filelist...

current behaviour of tasks using the filelist for non-existent files:

Concat: Log an error but contine
DependSet: Log an warning and continue
ExecuteOn: Pass on the missing file to the command
PathConvert: just described above
SubAnt -> Path: Log a warning and continue or throw an exception dependent on
parameter.
Touch: Create the file if it does not exist.

As copy already has the fail on error i'd say follow the SubAnt behaviour: Log a
warning and continue or throw an exception dependent on the failonerror parameter.

It doesn't seem to be straightforward to extend FileList in such a way that it
would fail / create file / remove from list / pass-on the non-existent files in
both a backward compatible and consistent way.

<copy> is slated to be modified to support all resource collection types.

fixed in 1.7 (HEAD)