c++ - Why does this not compile without enable_if? -

why following code not compile if remove enable_if?

template<class t, class = typename std::enable_if<              std::is_constructible<t, double,double,double>::value              >::type > operator t() const {     return t{x, y, z}; } 

example code: enable_if http://ideone.com/nwdmjh

without enable_if + compile error http://ideone.com/fbmamf

without enable_if, conversion operator exists every type. error if attempting instantiate conversion operator types cannot constructed {x, y, z}, doesn't change fact operator exists. because exists, type considered convertible double *, , overload resolution cannot pick best match.