adjacent_filtered lets the first element entry through

[Tony Lewis <tonyelewis@…>]

adjacent_filtered always lets the first element of the range leak through, even for a predicate that rejects that element as either argument. Eg.

int main() {
        const std::vector<int> a = { 0, 1, 2, 3, 4, 5 };

        auto b = a | boost::adaptors::adjacent_filtered( [] (const int &x, const int &y) {
                return ( ( x > 2 ) && ( y > 2 ) );
        } );

        for (const auto &x : b) {
                std::cerr << x << "\n";
        }
}

…outputs:

0
4
5

From what I can see in the code, the predicate is currently only applied in the increment() function, which leaves it too late for the first element to be checked.