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

[Akira Hatanaka via cfe-dev]

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
>> >
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>> >
>> >
>> >
>> > --
>> > Hal Finkel
>> > Assistant Computational Scientist
>> > Leadership Computing Facility
>> > Argonne National Laboratory
>> > _______________________________________________
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>> > cfe-dev at lists.llvm.org
>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>
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