Code shared from run.dlang.io.

main.d

import std.stdio;
import std.variant : Variant;

struct var {
    private Variant[string] values;

    @property
    Variant opDispatch(string name)() const {
        return values[name];
    }

    @property
    void opDispatch(string name, T)(T val) {
        values[name] = val;
    }
}

void main()
{
    var test;

    // Normal flow
    test.foo = "test";
    test.bar = 50;

    writeln("test.foo = ", test.foo);
    writeln("test.bar = ", test.bar);

    // Error flow
    with (test) {
        foobar = 3.14;
    }

    writeln(test.foobar);
}