Cannot use UFCS in lambda?

[berni via Digitalmars-d-learn]

The following program does not compile:

import std.stdio;
import std.algorithm;
struct A
{
   struct S
   {
       int x;
   }

   const bool foo(in S s, in int k)
   {
       return s.x<k;
   }

   void bar()
   {
       [S(1),S(2),S(3),S(4),S(5)].filter!(a=>a.foo(3)).writeln;
   }
}

void main()
{
   A().bar();
}

I get (using rdmd):

test.d(18): Error: no property foo for type S
/usr/include/dmd/phobos/std/algorithm/iteration.d(1108):        
instantiated from here: >FilterResult!(__lambda1, S[])
test.d(18):        instantiated from here: filter!(S[])

When I replace the UFCS-Syntax in the lambda by a function call 
it works:

[S(1),S(2),S(3),S(4),S(5)].filter!(a=>foo(a,3)).writeln;

Where is my mistake?