[cfe-users] variable template within class produces non-static data member cannot be constexpr

[Gabriel Dos Reis]

I don't understand the comment; but, I can tell that a class member
variable template must be declared static. 'constexpr'ness is immaterial to
that.

-- Gaby

On Thu, Nov 13, 2014 at 10:48 PM, David Blaikie <dblaikie at gmail.com> wrote:

> My guess is that the proposal was written assuming a certain
> implementation of constexpr that never panned out.
>
> It looks like both GCC and Clang expect constexpr member variables to be
> explicitly marked static:
>
> const.cpp:5:19: error: non-static data member 'f' declared 'constexpr'
>    constexpr foo f{};
>                    ^
>
> (is GCC 4.9's diagnostic - for a simple non-template constexpr member variable)
>
>
> On Thu, Nov 13, 2014 at 8:38 PM, Larry Evans <cppljevans at suddenlink.net>
> wrote:
>
>> The following code:
>>
>> //{=======================================
>> #include <type_traits>
>> struct variable_template
>> {
>>     template <int Value>
>>     std::integral_constant<int,Value> constexpr
>>   value{}
>>     ;
>> };
>> #include <iostream>
>> #include <iomanip>
>> int main()
>> {
>>     std::cout<<
>>       variable_template::
>>       value<1>()<<std::endl;
>>     return 0;
>> }
>> //}=======================================
>> produces error:
>> //{=======================================
>>
>> /home/evansl/dwnlds/llvm/3.5/binary/clang+llvm-3.5.0-x86_64-linux-gnu/bin/clang++
>> -c -O0 -g -stdlib=libc++  -std=c++14 -ftemplate-backtrace-limit=0
>> -I/home/evansl/prog_dev/clang/libcxx
>> -I/home/evansl/prog_dev/boost/sandbox/rw/non_variadic_templates
>> -I/home/evansl/prog_dev/boost/sandbox/rw/sandbox/lje
>> -I/home/evansl/prog_dev/boost/boost-releases/ro/boost_1_56_0
>> -DTYPE_AT_IMPL=0   -ftemplate-depth=324  variable_templates.cpp -MMD -o
>> /tmp/build/clangxx3_5_bin/clang/libcxx/variable_templates.o
>> variable_templates.cpp:23:39: error: non-static data member cannot be
>> constexpr; did you intend to make it static?
>>     std::integral_constant<int,Value> constexpr
>>                                       ^
>>                                       static
>> 1 error generated.
>> //}=======================================
>> when compiled with the binary clang downloaded from:
>>
>>
>>
>> http://llvm.org/releases/3.5.0/clang+llvm-3.5.0-x86_64-linux-gnu-ubuntu-14.04.tar.xz
>>
>> and used on system:
>>
>> Linux lje-OptiPlex-9020 3.13.0-39-generic #66-Ubuntu SMP Tue Oct 28
>> 13:30:27 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
>>
>> The code is, AFAICT, essentially the same as the `struct
>> matrix_measures' in section 3.2 of:
>>
>>   http://isocpp.org/files/papers/N3651.pdf
>>
>> Is this a clang bug or is the code somehow wrong?
>>
>> TIA.
>>
>> -regards,
>> Larry
>>
>>
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>
>
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