How is a Func<T> implicitly converted to Expression<Func<T>>?

I don't understand what is happening here:

Both of these lines compile:

 Func<object> func = () => new object();

 Expression<Func<object>> expression = ()=>new object();

But this doesn't:

 expression = func;

There isn't an implicit operator on LambdaExpression or Expression<TDelegate> that converts a delegate to the expression, so something else must be happening to make the assignment work. What is it?