[Bug c++/65327] New: GCC rejects "constexpr volatile int i = 5;"

[Casey at Carter dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65327

            Bug ID: 65327
           Summary: GCC rejects "constexpr volatile int i = 5;"
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Casey at Carter dot net

See
http://stackoverflow.com/questions/28883510/can-constexpr-be-combined-with-volatile
for discussion. GCC 5 (Identified as "5.0.0 HEAD 20150304" on Wandbox at
http://melpon.org/wandbox/permlink/u7BC3wFEfMv0zrVN) rejects this program:

int main() {
    constexpr volatile int i = 5;
}

with the diagnostic:

prog.cc: In function 'int main()':
prog.cc:3:28: error: both 'volatile' and 'constexpr' cannot be used here
    constexpr volatile int i = 5;

AFAICT, this program is well-formed: `constexpr` can be applied to literal
types, which includes scalar types, which includes cv-qualified arithmetic
types. Semantically, I *think* "constexpr volatile" is equivalent to "const
volatile" since "constexpr" objects are implicitly "const" but lvalue-to-rvalue
conversions applied to volatile glvalues cannot be part of a
core-constant-expression.