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

Tue May 31 17:13:18 PDT 2016

On Tue, May 31, 2016 at 5:00 PM, Hubert Tong via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> 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();
>

We should warn on both of these commas. If you give ContextRAII a
non-trivial destructor, we should warn on the first one but not the second.

> }
>
>
> 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
>> >
>> >
>> > _______________________________________________
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>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>
>>
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