forward-incompatible Function.prototype.toString requirement

[Michael Ficarra]

ES2015 section 19.2.3.5 (Function.prototype.toString) places four
restrictions on the output of Function.prototype.toString, one of which is

If the implementation cannot produce a source code string that meets these
> criteria then it must return a string for which *eval* will throw a
> *SyntaxError* exception.
>

What is a SyntaxError today may not be a SyntaxError tomorrow. How can an
implementation return a string that will satisfy this requirement in the
future when the language has added new syntax? Does the committee have a
SyntaxError recommendation that can be added as a non-normative note to
that section?

Michael Ficarra
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