What is this smiley-with-beard expression: "<:]{%>"?

Question

I came across the following program, which compiles without errors or even warnings:

int main(){
  <:]{%>; // smile!
}

Live example.

What does the program do, and what is that smiley-expression?

Answer 1

That's an empty lambda using a digraph disguise. Normal lambdas don't have beards.