[Bug c++/67333] New: [C++11][constexpr] constexpr functions incorrectly prohibit taking references to volatile types

[myriachan at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67333

            Bug ID: 67333
           Summary: [C++11][constexpr] constexpr functions incorrectly
                    prohibit taking references to volatile types
           Product: gcc
           Version: 5.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: myriachan at gmail dot com
  Target Milestone: ---

GCC 4.7.3 (at least) through GCC 6.0 reject the following due to "meow has
side-effects":

#include <cstddef>
#include <type_traits>

template <typename T, std::size_t S>
constexpr std::size_t lengthof(const volatile T (&)[S])
{
    return S;
}

int main()
{
    volatile int meow[4];
    static_cast<void>(meow); // shut up warning
    return static_cast<int>(std::integral_constant<std::size_t,
        lengthof(meow)>::value);
}

I believe that this is legal per [expr.const] in the Standard, because the
volatile parameter is never used in an lvalue-to-rvalue conversion, which is
what [expr.const] disallows in constant-expressions for volatile types.  Clang
versions that understand this code accept it; Visual C++ 2015 does as well.

Someone who replied to my question on the "std-discussion" mailing list
suggested that this is also technically legal as well:

#include <type_traits>

constexpr int Test(int x)
{
    volatile int v = x;
    return x;
}

int main()
{
    return std::integral_constant<int, Test(2)>::value;
}

GCC also rejects this, but Clang accepts this as well.  Any attempt to read v
will fail, though, so Clang is enforcing the rule.  I'm not on my Windows
machine as I write this, so I can't check MSVC.