[Bug c++/54021] New: [c++0x] __builtin_constant_p should be constexpr

["luto at mit dot edu" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54021

             Bug #: 54021
           Summary: [c++0x] __builtin_constant_p should be constexpr
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: luto@mit.edu


It's hard to tell how __builtin_constant_p works right now, due to PR54020.

// Preliminaries.
extern int nonconst_func(int);
constexpr int identity(int x) { return x; }
constexpr int zero() { return identity(0); }
constexpr int one() { return identity(1); }

// These are the same.  Only the latter is accepted, though.
// I suspect that the acceptance of the latter is due to the bug above.
constexpr int rejected_const_4(int x) { return __builtin_constant_p(x) ? 4 :
nonconst_func(x); }
constexpr int accepted_const_4(int x) { return
identity(__builtin_constant_p(x)) ? 4 : nonconst_func(x); }

// This is rejected.  I would like it to work.
constexpr int four = accepted_const_4(1);

The ability to use the construction __builtin_constant_p(x) ? const_func(x) :
nonconst_func(x) in constexpr context would be quite powerful.