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

Thu May 26 16:55:42 PDT 2016

> On 2016-May-26, at 16:16, Akira Hatanaka <ahatanak at gmail.com> wrote:
> 
> On Thu, May 26, 2016 at 3:58 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> 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 effectively allows lambdas in array bounds.
> 
> Akira, what does the patch for this look like?
> 
> 
> My first patch just replaced the call to ParseConstantExpresssion at ParseDecl.cpp:6106 with ParseExpression. I didn't see the error message about lambda after applying the patch. It also caused clang to accept expressions like this, if I remember correctly:
> 
> char a[1,2]; 

Hmm.  That would merit a warning.  IMO, -Wcomma should fire on every
use of the built-in comma operator that's not in the "increment"
statement of a for loop... I'm not sure if others agree though.

> >
> >  -Hal
> >
> > -- James
> >
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> >
> >
> > --
> > Hal Finkel
> > Assistant Computational Scientist
> > Leadership Computing Facility
> > Argonne National Laboratory
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