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

Tue May 31 14:46:55 PDT 2016

On Tue, May 31, 2016 at 5:28 PM, James Grosbach via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

>
> On May 26, 2016, at 4:55 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com>
> wrote:
>
>
> 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.
>
>
> I’m curious what that would look like on a large codebase. It sounds good
> to me, but potentially very noisy and perhaps a step too far in the
> direction of style enforcement rather than bug finding.
>
As it is, I find -Wlogical-op-parentheses and -Wmismatched-tags to be too
noisy already.

>
> A case I’d like to make sure gets caught is:
>
> int foo();
> int bar();
> ...
> a = foo(), (void)bar();
>
> There’s no warning or error for this code. However, change the assignment
> to a “return” and it’s a hard error. Precedence is a pain.
>
> The real issue is the far more nefarious:
> a = foo(), bar();
>
> It’s going to be pretty darn rare to see that and the actual results being
> what was intended by the author.
>
>
>
>
>
> -Hal
>
> -- James
>
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>
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
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