[cfe-dev] c++ question: can lambda be used in VLA?

Thu May 26 16:03:36 PDT 2016

> On 2016-May-26, at 16:02, Hal Finkel <hfinkel at anl.gov> wrote:
> 
> ----- Original Message -----
>> From: "Duncan P. N. Exon Smith" <dexonsmith at apple.com>
>> To: "Hal Finkel" <hfinkel at anl.gov>
>> Cc: "James Dennett" <james.dennett at gmail.com>, "Akira Hatanaka" <ahatanak at gmail.com>, "Richard Smith"
>> <richard at metafoo.co.uk>, "Clang Dev" <cfe-dev at lists.llvm.org>
>> Sent: Thursday, May 26, 2016 5:58:04 PM
>> Subject: Re: [cfe-dev] c++ question: can lambda be used in VLA?
>> 
>> 
>>> On 2016-May-25, at 16:41, Hal Finkel via cfe-dev
>>> <cfe-dev at lists.llvm.org> wrote:
>>> 
>>> 
>>> From: "James Dennett via cfe-dev" <cfe-dev at lists.llvm.org>
>>> To: "Akira Hatanaka" <ahatanak at gmail.com>
>>> Cc: "Richard Smith" <richard at metafoo.co.uk>, "Clang Dev"
>>> <cfe-dev at lists.llvm.org>
>>> Sent: Wednesday, May 25, 2016 6:37:46 PM
>>> Subject: Re: [cfe-dev] c++ question: can lambda be used in VLA?
>>> 
>>> On Wed, May 25, 2016 at 4:19 PM, Akira Hatanaka
>>> <ahatanak at gmail.com> wrote:
>>> On Tue, May 24, 2016 at 7:11 PM, James Dennett
>>> <james.dennett at gmail.com> wrote:
>>> On Tue, May 24, 2016 at 6:25 PM, Akira Hatanaka via cfe-dev
>>> <cfe-dev at lists.llvm.org> wrote:
>>> I wasn't requesting that clang accept lambda expressions used for
>>> array bounds but was asking whether it was valid in c++. Is this
>>> something that is open to interpretation because it's not covered
>>> by the standard?
>>> 
>>> FYI, this isn't something that I made up. It was in a code a user
>>> wrote.
>>> 
>>> 
>>> It's covered by the standard, and as Clang's error message says,
>>> lambdas are not allowed in constant expressions in C++11 or C++14.
>>> 
>>> 
>>> Yes, the c++ standard gives a list of subexpressions that are not
>>> allowed in constant expressions and lambda expression is one of
>>> them.
>>> 
>>> This doesn't seem to apply to C99's extension for variable length
>>> arrays because array sizes are not required to be constant
>>> expressions.
>>> 
>>> 
>>> I was replying to you saying that you were "asking whether it was
>>> valid in C++", and whether "it's not covered by the standard".
>>> 
>>> C99 doesn't have lambdas, so it doesn't allow this.  C++ doesn't
>>> have VLAs, so it doesn't allow it.
>>> 
>>> The de facto language accepted by Clang doesn't accept it, as you
>>> already noted.
>>> 
>>> There's no specification that tells us what the "right thing to do"
>>> is here.  We could extend Clang to support this non-standard
>>> combination of C99 with C++11, and it might even make it a little
>>> more consistent, but if it adds any implementation complexity then
>>> it may not be worthwhile to support a corner case that's not
>>> allowed by any language standard.
>>> What did the most recent wording for C++ ARBs say about this issue?
>> 
>> The latest version I could find is here:
>> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3820.html#Introduction
>> 
>> The changes to 8.3.4 Arrays [dcl.array] change the argument from a
>> constant-expression_opt to an expression_opt:
>> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3820.html#dcl.array
>> 
>> I think the Array TS was killed in Jacksonville due to lack of
>> interest,
> 
> Unfortunately; but not exactly for lack of interest. As far as I can tell, there was a lack of progress on the second more-C++-like companion part that was part of the compromise to get the ARBs.

Ah.

> -Hal
> 
>> but the interaction between these features seems
>> straightforward to me.  When the C++ language extension for VLAs is
>> turned on, we shouldn't treat the array argument as a
>> constant-expression.  This effectively allows lambdas in array
>> bounds.
>> 
>> Akira, what does the patch for this look like?
>> 
>>> 
>>> -Hal
>>> 
>>> -- James
>>> 
>>> _______________________________________________
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>>> 
>>> 
>>> 
>>> --
>>> Hal Finkel
>>> Assistant Computational Scientist
>>> Leadership Computing Facility
>>> Argonne National Laboratory
>>> _______________________________________________
>>> cfe-dev mailing list
>>> cfe-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>> 
>> 
> 
> -- 
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory