On 12/08/2020 20.34, Uecker, Martin wrote:
> Am Mittwoch, den 12.08.2020, 09:26 -0700 schrieb Richard Smith:
>> 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).

Agreed. It also seems easier to me to simply introduce an optional
sequence of labels at the end of a compound-statement, instead of
a large-ish refactoring of the C++ grammar.

> I don't quite understand why wording changes would
> be necessary. Can you explain?

In C++, we say in [stmt.goto]
"The goto statement unconditionally transfers control to the statement labeled by the identifier."

This is obviously not true for labels at the end of a compound-statement,
because there is no statement that is labeled in that case.

So, we need a wording change here.

Jens