Odd behaviour with istream_iterator

  I'm using gcc 3.3.5. This code:

std::set<std::string> t(std::istream_iterator<std::string>(std::cin),
                        std::istream_iterator<std::string>());

gives a strange error message:

error: cannot use `::' in parameter declaration

  If I try it this way:

std::istream_iterator<std::string> is(std::cin);
std::istream_iterator<std::string> eof();
std::set<std::string> t(is, eof);

it also gives an error:

error: no matching function for call to `std::set<*snip*>::set(*snip*)'

  Strangely enough, this:

std::set<std::string> t((std::istream_iterator<std::string>(std::cin)),
                        std::istream_iterator<std::string>());

compiles and works just fine. The only difference is the extra
parentheses around the first parameter. I can't get the second version
above to compile regardless of any amount of parentheses anywhere.

  Even more strangely, if I add the extra parentheses around the
second parameter, like this:

std::set<std::string> t((std::istream_iterator<std::string>(std::cin)),
                        (std::istream_iterator<std::string>()));

it once again doesn't compile:

error: syntax error before `)' token

  I don't understand this. What's going on?