On Wed, 12 Aug 2020, 04:26 Uecker, Martin via Liaison, <
liaison_at_[hidden]> wrote:

> Am Dienstag, den 11.08.2020, 17:29 +0200 schrieb Jens Maurer:
> > On 11/08/2020 08.37, Uecker, Martin via Liaison wrote:
> > >
> > > Hi all,
> > >
> > > I recently proposed changes to the C grammar which got
> > > voted into C2X by WG14 (N2508, also see N2496).
> > >
> > > This allows placing of labels everywhere in a compound
> > > statement, even before declarations and at the end
> > > of a block, which was not possible in C so far.
> > >
> > > Example:
> > >
> > > {
> > > start:
> > > int i;
> > > mid:
> > > int j;
> > > end:
> > > }
> > >
> > > It has been pointed out to me that while C++ allows
> > > labels before declarations, it does not allow them
> > > at the end of a compound statement.
> > >
> > > I plan to propose a change to C++ to make this
> > > possible too and I wonder if you have any comments
> > > or suggestions related to this?
> >
> > I'm lukewarm on this. Your example becomes valid C++
> > by adding a ";" after the "end:".
> >
> > So, having the change does not materially alter the
> > expressiveness of the common subset of C++ and C.
>
> It would enhance compatibility.
>
> > I'd like to see the wording change for C++ to understand
> > whether it's likely low-cost overall to add this.
>
> Ok, no wording yet, but the suggested grammar change would
> be as follows (omitting attributes for simplicity):
>
> 8.1(1) gets refactored from
>
> statement:
> labeled-statement
> expression-statement
> compound-statement
> selection-statement
> iteration-statement
> jump-statement
> declaration-statement
> try-block
>
> to the following:
>
> statement:
> labeled-statement
> unlabeled-statement
> declaration-statement
>

The separation of declaration-statement here seems arbitrary (and incorrect
without changes elsewhere, since we specify execution semantics for
declaration-statements that don't apply to declarations in general, in
particular registering atexit destructors for static locals).

I would suggest you move declaration-statement into unlabeled-statement and
remove declaration from statement-seq-item.

Alternatively, a more isolated change: add an optional sequence of labels
to the end of a compound statement.

compound-statement:
    { statement-seq <ins>label-seq[opt]</ins> }

unlabeled-statement:
> expression-statement
> compound-statement
> selection-statement
> iteration-statement
> jump-statement
> try-block
>
>
>
> 8.2(1) gets refactored from
>
> labeled-statement:
> identifier : statement
> case constant-expression : statement
> default : statement
>
> to:
>
> labeled-statement:
> label statement
>
> label:
> identifier :
> case constant-expression :
> default :
>
>
> In 8.4(1), we then do the actual
> change which is changing
>
>
> compound-statement:
> statement-seq
>
> statement-seq:
> labeled-statement
> statement-seq statement
>
> to
>
> compound-statement:
> statement-seq
>
> statement-seq:
> statement-seq-item
> statement-seq statement-seq-item
>
> statement-seq-item:
> label
> unlabeled-statement
> declaration
>
>
>
> Best,
> Martin
>
>
>
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