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

Thu May 26 15:58:04 PDT 2016

> 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 effectively allows lambdas in array bounds.

Akira, what does the patch for this look like?

> 
>  -Hal
> 
> -- James
> 
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> -- 
> Hal Finkel
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> Leadership Computing Facility
> Argonne National Laboratory
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