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.
