cannot deduce template lambda from argument

[aliak via Digitalmars-d-learn]

Hi,

Having a little trouble understanding lambda type deduction. I 
have this lambda:

immutable lambda(T) = (T n) => n * n;

and if I call it with an explicit type it works else it errors 
with: lambda cannot deduce function from argument types !()(int)

auto x = lambda!int(2); // ok
auto x = lambda(2); // errors

But if it's a normal template function then calling it without 
explicit type is ok.

Thanks for any help!