id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
9953	lazy_enable_if is not sfinae friendly	hui.li@…	Peter Dimov	"The following code fails to compile, even though boost::result_of itself is sfinae friendly.
{{{
template < typename T >
typename boost::lazy_enable_if_c<
  boost::is_class<T>::value,
  boost::result_of<T(int)>
>::type
test(int){}

template < typename >
void test(...) {}

struct A{}; // boost::result_of<A(int)> is a valid type that doesn't have a member called ""type""

int main(int argc, const char * argv[])
{
  test<A>(0); // test<A>(int) meant to be a substitution failure
  return 0;
}
}}}
I'm not sure if this is a bug/flaw, or a lack of feature, or maybe this kind of use is deliberately disallowed for some reason, but it could be very useful if {{{lazy_enable_if_c<bool B, typename T>}}} etc support uses when {{{typename T::type}}} itself may cause a substitution failure.

Proposing to change implementation of {{{lazy_enable_if_c}}} and {{{lazy_disable_if_c}}} to:
{{{
template <bool B, class T>
struct lazy_enable_if_c : T {}; // inherits from T

template <class T>
struct lazy_enable_if_c<false, T> {};

template <bool B, class T = void>
struct disable_if_c : T {};  // inherits from T

template <class T>
struct disable_if_c<true, T> {};
}}}

"	Feature Requests	new	To Be Determined	core	Boost 1.55.0	Problem