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

Tue May 31 17:00:51 PDT 2016

The comma operator is rather useful for RAII objects and to disambiguate
the vexing parse in favour of the expression.

For example (note: Clang does not handle this correctly; PR on the way):
struct ContextRAII {
   ContextRAII(int);
};

int x[100];

void bar();

void foo(int idx) {
   ContextRAII(x[0, idx]), bar();
}

On Tue, May 31, 2016 at 6:04 PM, Duncan P. N. Exon Smith <
dexonsmith at apple.com> wrote:

>
> > On 2016-May-31, at 14:46, Hubert Tong <hubert.reinterpretcast at gmail.com>
> wrote:
> >
> > 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.
>
> I agree that -Wlogical-op-parentheses is fairly noisy, but IMO this
> would generate a *lower* amount of noise.  I think very few people
> actually use `,`, whereas `&&` and `||` are frequently used.
>
> > 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
> >>>>
> >>>> _______________________________________________
> >>>> cfe-dev mailing list
> >>>> cfe-dev at lists.llvm.org
> >>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> >>>>
> >>>>
> >>>>
> >>>> --
> >>>> Hal Finkel
> >>>> Assistant Computational Scientist
> >>>> Leadership Computing Facility
> >>>> Argonne National Laboratory
> >>>> _______________________________________________
> >>>> cfe-dev mailing list
> >>>> cfe-dev at lists.llvm.org
> >>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> >
> >
> > _______________________________________________
> > cfe-dev mailing list
> > cfe-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>
>
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