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

Thu Jun 9 10:26:43 PDT 2016

I've sent a patch to cfe-commtis for review. Please take a look:

http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20160606/161536.html

I think it makes the handling of VLAs more consistent without adding too
much complexity to the existing code.

On Thu, Jun 9, 2016 at 9:21 AM, Akira Hatanaka <ahatanak at gmail.com> wrote:

>
>
> On Tue, May 31, 2016 at 5:15 PM, Richard Smith <richard at metafoo.co.uk>
> wrote:
>
>> On Thu, May 26, 2016 at 3:58 PM, Duncan P. N. Exon Smith via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>>
>>>
>>> > 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,
>>> 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 isn't quite that simple... We do not have a C++ language extension
>> for VLAs that can be turned on and off in the way you seem to expect. What
>> we have is an extension warning that can be disabled or turned into an
>> error. Following our usual philosophy for warning flags, it's not
>> reasonable for -Wno-vla to affect how we parse an array bound.
>>
>>
>
> If it's not desirable to change the how we parse an array bound, I'll just
> add a flag to ExpressionEvaluationContextRecord that indicates whether an
> array bound in a BlockContext is being parsed.
>
>
>>   This effectively allows lambdas in array bounds.
>>>
>>> Akira, what does the patch for this look like?
>>>
>>> >
>>> >  -Hal
>>> >
>>> > -- James
>>> >
>>> > _______________________________________________
>>> > 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
>>> > _______________________________________________
>>> > cfe-dev mailing list
>>> > cfe-dev at lists.llvm.org
>>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>
>>> _______________________________________________
>>> cfe-dev mailing list
>>> cfe-dev at lists.llvm.org
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>>>
>>
>>
>
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