Restarting this thread, because it's still an issue a year+ on.

In the Font Loading spec <http://dev.w3.org/csswg/css-font-loading/>,
I have a FontFaceSet interface
<http://dev.w3.org/csswg/css-font-loading/#FontFaceSet-interface>,
which is a Set that contains FontFace objects, plus a few other
methods and attributes.

Except that it's not a Set, because I can't subclass Sets reliably.
Instead, I have to duplicate the entire Set interface *as it exists
today*, and have it delegate to a hidden internal Set object.  This
means that when TC39 extends the Set interface, FontFaceSet will have
to be manually updated and bugfixed to contain its own version of the
new methods.  It also means that when authors define new methods on
Set.prototype, they don't apply to FontFaceSet objects unless they
manually copy them to FontFaceSet.prototype as well.

This is obviously bad.  But I can't just directly subclass Set.  If I
do, then authors can do Set.prototype.add(ffset, "hahaha, I'm a
string, not a font face!"), and then the Set is corrupted.  I could,
of course, defensively write all the FFS operations to check each
entry for being a FontFace object first, but that's not quite enough,
because *authors* also have to do this when iterating a FFS.

It's accepted and idiomatic for all sorts of methods in the web
platform to do typechecks on their arguments, and throw errors when
they don't match what's expected.  It's impossible for me to robustly
do this for the Set and Map methods, though.  Can we come up with
something that allows me to enforce these kinds of checks, like I'm
currently doing with my Set-lookalike, and like we do everywhere else
in the entire web platform?

~TJ

Restarting this thread, because it's still an issue a year+ on.

In the [Font Loading spec](http://dev.w3.org/csswg/css-font-loading/), I have a [FontFaceSet interface](http://dev.w3.org/csswg/css-font-loading/#FontFaceSet-interface), which is a Set that contains FontFace objects, plus a few other methods and attributes.

Except that it's not a Set, because I can't subclass Sets reliably.
Instead, I have to duplicate the entire Set interface *as it exists
today*, and have it delegate to a hidden internal Set object.  This
means that when TC39 extends the Set interface, FontFaceSet will have
to be manually updated and bugfixed to contain its own version of the
new methods.  It also means that when authors define new methods on
Set.prototype, they don't apply to FontFaceSet objects unless they
manually copy them to FontFaceSet.prototype as well.

This is obviously bad.  But I can't just directly subclass Set.  If I
do, then authors can do `Set.prototype.add(ffset, "hahaha, I'm a string, not a font face!")`, and then the Set is corrupted.  I could,
of course, defensively write all the FFS operations to check each
entry for being a FontFace object first, but that's not quite enough,
because *authors* also have to do this when iterating a FFS.

It's accepted and idiomatic for all sorts of methods in the web
platform to do typechecks on their arguments, and throw errors when
they don't match what's expected.  It's impossible for me to robustly
do this for the Set and Map methods, though.  Can we come up with
something that allows me to enforce these kinds of checks, like I'm
currently doing with my Set-lookalike, and like we do everywhere else
in the entire web platform?