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

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?

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

Stack Overflow Q#15736282, score: 301