http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60511
Bug ID: 60511
Summary: [C++1y][N3652] Missing extended constexpr function
support
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: daniel.kruegler at googlemail dot com
gcc 4.9.0 20140309 (experimental) compiled with the flags
-std=c++1y -Wall -pedantic
rejects:
//----------------------
template<class T>
struct L
{
T t;
constexpr L(const T& t) : t(t) {}
constexpr const T& value() const & { return t; }
constexpr T& value() & { return t; }
};
template<class T>
constexpr T twice(const T& t) {
L<T> l(t);
l.value() *= 2;
return l.value();
}
int main() {
constexpr int i = twice(12);
static_assert(i == 24, "");
}
//----------------------
<quote>
prog.cc:7:16: error: 'constexpr T& L<T>::value() const &' cannot be overloaded
constexpr T& value() & { return t; }
^
prog.cc:6:22: error: with 'constexpr const T& L<T>::value() const &'
constexpr const T& value() const & { return t; } ^
prog.cc: In instantiation of 'constexpr T twice(const T&) [with T = int]':
prog.cc:18:29: required from here
prog.cc:13:13: error: assignment of read-only location 'l.L<T>::value<int>()'
l.value() *= 2;
^
prog.cc:15:1: error: body of constexpr function 'constexpr T twice(const T&)
[with T =
int]' not a return-statement
}
^
prog.cc: In function 'int main()':
prog.cc:18:29: error: 'constexpr T twice(const T&) [with T = int]' called in a
constant expression
constexpr int i = twice(12);
^
prog.cc:11:13: note: 'constexpr T twice(const T&) [with T = int]' is not usable
as
a constexpr function because:
constexpr T twice(const T& t) {
^
</quote>
The compiler seems currently not apply the extended C++14 constexpr rules as of
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3652.html
1) constexpr makes non-static member functions automatically const member
functions
2) constexpr functions require a single return statement
3) constexpr functions reject mutable operations
