[C++ Patch] PR 51911 V2 ("G++ accepts new auto { list }")

[Paolo Carlini]

Hi,

On 09/11/2015 10:05 PM, Jason Merrill wrote:
> On 09/11/2015 03:11 PM, Paolo Carlini wrote:
>> this is a slightly reworked (simplified) version of a patch I sent a
>> while ago. The issue is that we are not enforcing at all 5.3.4/2 in the
>> parser, thus we end up rejecting the first test below with a misleading
>> error message talking about list-initialization (and a wrong location),
>> because we diagnose it too late like 'auto foo{3, 4, 5};', and simply
>> accepting the second. Tested x86_64-linux.
>
> Hmm, I think we really ought to accept
>
>   new auto { 2 }
>
> to be consistent with all the other recent changes to treat { elt } 
> like (elt); this seems like a piece that was missed from DR 1467.  Do 
> you agree, Ville?
I see, while waiting for Ville, maybe I can ask what we should do in 
case he agrees. The error message we currently emit for new auto { 3, 4, 
5 } seems suboptimal in various ways:

51911.C:6:31: error: direct-list-initialization of ‘auto’ requires 
exactly one element [-fpermissive]
  auto foo = new auto { 3, 4, 5 };
                                ^
51911.C:6:31: note: for deduction to ‘std::initializer_list’, use 
copy-list-initialization (i.e. add ‘=’ before the ‘{’)

the caret is under the last '}' and the note doesn't make much sense (of 
course do_auto_deduction doesn't know we are handling a new). Thus I 
wonder if we should anyway have something in the parser and with which 
exact wording (just tweak what I sent earlier replacing 'exactly one 
parenthesized expression' with 'exactly one element'?!?)

Thanks,
Paolo.