enum Type
{
    A,
    B,
}

struct Foo {}
struct Bar
{
    Foo foo;
    alias foo this;
}

void main() 
{
    Bar bar;
    test!(Type.A)(bar);           // fail
    test!(Type.A)(cast(Foo)bar);  // ok
}

void test(Type type)(Foo foo)
    if (type == Type.A)
{
}

void test(Type type)(Foo foo)     // comment out and both run ok
    if (type == Type.B)
{
}

If you remove the second test() template both instantiations in main() will work, which is very odd.