compare struct with string member is wield;

[Cheng Wei]

Sorry. The previous is not the one causes the problem.
Try this:

struct S {
    string str = "Hello";   // Adding an initial value here.
};

S g_s;

unittest {
    S s1;
    S s2;
    assert(s1 == s2); // Success
    assert(g_s == s1); // Fail
    auto s3 = g_s;
    assert(s3 == g_s);; // Even this will fail.
}

It seems if the string is initialized with a default value, then this
does not work.