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: unassig...@gcc.gnu.org
ReportedBy: l...@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.
