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

Tue May 31 14:28:40 PDT 2016

> 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.

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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