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

[Richard Smith via cfe-dev]

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.


>   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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