Example test framework

gistfile1.d

auto test(T)(T lhs) {
    auto t = new Test!T;
    t.lhs = lhs;
    import std.typecons : Tuple;
    return Tuple!(Test!T , "lhs", Test!T, "verify")(t, t);

}

class Test(T) {
    T lhs;
    string testError;
    string testFile;
    size_t testLine;

    bool opEquals(T)(auto ref const T rhs, in string file = __FILE__, in size_t line = __LINE__) {
        import std.string : format;
        testError = format("lhs = %s; rhs = %s;", lhs.m, rhs.m);
        testFile = file;
        testLine = line;
        return lhs == rhs;
    }

    auto opCall(bool d) {
        if(!d)
            throw new Exception(testError, testFile, testLine);
    }
}

unittest {
    struct A {
        int m;
    }

    A a;
    A b;
    a.m = 5;
    b.m = 5;

    auto testA = a.test;
    with(testA) {
        verify(testA.lhs == b);
        b.m = 6;
        verify(testA.lhs != b);
        verify(testA.lhs == b);
    }
}