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,,,