Overloading does not work with user-defined structs when there are two overloaded functions receiving the struct (one by ref and one by value).

struct MyStruct {}

void fun(ref MyStruct a) {}
void fun(MyStruct a) {}

unittest
{
    MyStruct a;
    fun(a); // fails both match
}

Tested with DMD32 D Compiler v2.057 which results in the error message:
Error: function struct.fun called with argument types:
        ((MyStruct))
matches both:
        struct.fun(ref MyStruct a)
and:
        struct.fun(MyStruct a)

But fun(ref MyStruct a) is more specialized. Thus, it should win.
The overloading protocol is defined in http://dlang.org/function.html section "Function Overloading" and in TDPL Section 5.5 "Overloading". Both description (though different in wording) specify the same behavior.

It works for int. And I assume for other built-in types as well. But I haven't checked.

I believe this issue is important. Because it is often needed (as an optimization to avoid the copy/move) for e.g. operator overloading.