if auto and method call

[Stanislav Blinov via Digitalmars-d-learn]

Would be prettier as a language feature, but still:

template test(alias pred)
{
     import std.functional : unaryFun;
     alias P = unaryFun!pred;

     auto test(R)(R r)
     {
         struct Test
         {
             R v;

             string toString()
             {
                 import std.conv : to;
                 return v.to!string;
             }

             bool opCast(B : bool)() const
             {
                 return P(v);
             }

             ref inout(R) get() inout
             {
                 return v;
             }

             alias get this;
         }

         import std.algorithm : move;
         return Test(move(r));
     }
}

void main()
{
     import std.regex;
     import std.stdio;
     string s1 = "hello";
     string s2 = "world";

     if (auto x = test!"!a.empty"(match(s1, "hello")))
     {
         writeln(x.captures[0]);
     }

     if (auto x = test!"!a.empty"(match(s2, "hello")))
     {
         writeln(x.captures[0]);
         assert(0);
     }

     // UFCS:

     if (auto x = s1.match("world").test!"!a.empty")
     {
         writeln(x.captures[0]);
         assert(0);
     }

     if (auto x = s2.match("world").test!"!a.empty")
     {
         writeln(x.captures[0]);
     }

     if (auto x = 3.test!"a == 3")
     {
         writeln(x, " equals 3, huh?");
     }
}