/+dub.sdl:

dependency "sumtype" version="~>0.5"

+/

​

template acceptOnly(Args...)

    if (Args.length >= 2)

{

    alias fun = Args[$ - 1];

    alias Types = Args[0 .. $ - 1];

​

    auto acceptOnly(T)(T arg)

    {

        import std.meta: staticIndexOf;

​

        static assert(staticIndexOf!(T, Types) >= 0,

            "Invalid argument type `" ~ T ~ "`; must be one of: " ~

            Types.stringof);

​

        return fun(arg);

    }

}

​

void main()

{

    import sumtype;

    import std.stdio: writeln;

    SumType!(int, float, string) x = 123;

    x.match!(

        acceptOnly!(int, float,

            num => writeln("number")

        ),

        (string _) => writeln("string")

    );

}