Single call-by-name constructor arg not required?

Why does line #2 below compile? It's (seemingly) not giving the required constructor arg.

class F(x: => Unit) {}
new F                     // Compiles (strange)
def f(x: =>Unit) = ()
f                         // Does not compile (good)

Is it permitted sugar just for this special case? Can you point to docs on the subject? I'm using Scala 2.9.0.