Allowing escape sequences to be synthesized would be a surprising change in
behavior. If we were designing this de novo it's just a choice, but the
preprocessor is old, shared with C, and baked into lots of tools.

On Mon, Jan 4, 2021 at 9:54 AM Peter Brett via SG16 <sg16_at_[hidden]>
wrote:

> Please could someone remind me of the *downsides* of allowing escape
> sequences to be synthesized into string literals through pre-processor
> concatenation?
>
> Many thanks,
>
> Peter
>
> > -----Original Message-----
> > From: SG16 <sg16-bounces_at_[hidden]> On Behalf Of Jens Maurer via
> SG16
> > Sent: 19 December 2020 22:45
> > To: Corentin Jabot <corentinjabot_at_[hidden]>; SG16 <
> sg16_at_[hidden]>
> > Cc: Jens Maurer <Jens.Maurer_at_[hidden]>
> > Subject: Re: [SG16] Handling literals throughout the translation phases
> >
> > EXTERNAL MAIL
> >
> >
> > On 18/12/2020 10.33, Corentin Jabot wrote:
> > > On Thu, Dec 17, 2020, 22:33 Jens Maurer via SG16 <
> sg16_at_[hidden]
> > <mailto:sg16_at_[hidden]>> wrote:
> > >
> > >
> > > I'm working on a paper that switches C++ to a modified "model B"
> > approach for
> > > universal-character-names as described in the C99 Rationale v5.10,
> > section 5.2.1.
> > >
> > >
> > > I thought sg16 agreed to not replace ucn until phase 5 a few meetings
> ago,
> > did I completely missunderstood what sg16 agreed ?
> >
> > The difference is that we do not produce UCNs is phase 1.
> > Instead, phase 1 simply produces Unicode scalar values.
> > Any UCNs that appeared in the original source are replaced later.
> >
> > > My current idea is to focus on the creation of the string literal
> > > object; that's when transcoding to execution (literal) encoding
> > > happens. All other uses of string-literals don't produce objects,
> > > so aren't transcoded.
> > >
> > > In order to be able to interpret escape-sequences in phase 5/6,
> > > we need a "tunnel" for numeric-escape-sequences. One idea would
> > > be to add "code unit characters" to the translation character set,
> > > where each such character represents a code unit coming from a
> > > numeric-escape-sequence. The sole purpose is to keep the
> > > code units safe until we produce the initializer for the
> > > string literal object.
> > >
> > > The alternative would be to delay all interpretation of escape-
> > > sequences to when we produce the initializer for the string
> > > literal object, but that also means we need to delay string
> > > literal concatenation until that time (see first item above).
> > >
> > >
> > > Would that cause any issue? This would otherwise be my preferred
> solution!
> >
> > We currently support operator "" "" "" in [over.literal], for
> example.
> > We'd need to make string-literal concatenation first-class citizens
> > in phase 7 (e.g. making it a constant expression or so), which is a
> fairly
> > large hammer.
> >
> > Jens
> >
> >
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