Consider the folowing:
class XXX {
public:
XXX(char) {}
template <typename T> XXX(T) = delete;
};
fprintf(stderr, "ic<char>::value = %i\n",
std::is_convertible<char, XXX>::value);
fprintf(stderr, "ic<double>::value = %i\n",
std::is_convertible<double, XXX>::value);
The code above doesn't compile on gcc 4.6:
In file included from /home/piotr.wyderski/f1/base/common.h:9:0,
from /home/piotr.wyderski/f1/main.cpp:8:
/opt/gcc-trunk-old2/bin/../lib/gcc/i686-pc-cygwin/4.6.0/include/c++/type_traits:
In instantiation of `const bool std::__is_convertible_helper<double, XXX, false
>::__value':
/opt/gcc-trunk-old2/bin/../lib/gcc/i686-pc-cygwin/4.6.0/include/c++/type_traits:
309:12: instantiated from `std::is_convertible<double, XXX>'
/home/piotr.wyderski/f1/main.cpp:219:81: instantiated from here
/home/piotr.wyderski/f1/main.cpp:186:31: error: deleted function `XXX::XXX(T) [w
ith T = double]'
/opt/gcc-trunk-old2/bin/../lib/gcc/i686-pc-cygwin/4.6.0/include/c++/type_traits:
302:71: error: used here
IMO it should result in an executable program which prints:
ic<char>::value =1
ic<double>::value = 0
as double is obviously not convertible to XXX -- the "template
<typename T> XXX(T) = delete;"
idiom: is very similar to an example from the N2346 proposal:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2346.htm
"Removing dangerous conversions is as important as removing undesired
language defaults.
struct type
{
type( long long ); // can initialize with an long long
type( long ) = delete; // but not anything less
};
extern void bar( type, long long ); // and the same for bad overloads
void bar( type, long ) = delete; // of free functions
"
Should I file a bug report or is it broken by design?
A proposed fix is to remove the current template-based detector in the
standard library and provide
a built-in function __is_convertible which would check such corner
cases in a much smarter way
than is possible with templates:
http://gcc.gnu.org/onlinedocs/gcc-4.4.2/gcc/Type-Traits.html#Type-Traits
Best regards,
Piotr Wyderski
