>
> - Was EWG informed that the proposal can be understood as introducing new
> core language undefined behaviour for existing programs with identifiers in
> NFC form where the concatenation is not in NFC form ([cpp.concat])? I note
> that R1 of the paper was not clear on that point and R2 does not identify
> it as a consideration.
>
> I don't recall discussing this in EWG, but I believe it was discussed at
> some point and the conclusion was that an identifier not in NFC form
> resulting from such concatenation was ill-formed. Can you elaborate as to
> how the UB manifests?
>
The typical manifestation of the UB in cpp.concat that does not produce a
diagnostic for the case in question is that the concatenation may instead
leave the operand tokens behind. That is, the ill-formed result is not
required to manifest.