The following code does not compile but it should.

ufcs.d:
struct Foo
{
   int _member;
   alias _member this;
}

int foo(Foo f) { return f._member; }

unittest
{
   Foo f;
   f.foo(); // fails to compile
}

Using dmd2.059 you get:
Error: function ufcs.foo (Foo f) is not callable using argument types (int)
Error: cannot implicitly convert expression (f._member) of type int to Foo

Note, that this bugs prevents using UFCS with std.typecons.Tuple.

http://forum.dlang.org/thread/mailman.1925.1334827762.4860.digitalmars-d@puremagic.com