Given a type 
T and a subexpression 
E of type 
U,
if 
U models
derived_from<basic_istream<typename U::char_type,
typename U::traits_type>>,
then the expression 
views::istream<T>(E) is expression-equivalent to
basic_istream_view<T, typename U::char_type,
typename U::traits_type>(E);
otherwise, 
views::istream<T>(E) is ill-formed
.