After ripping my hair out for several hours trying to get a more complex piece of code to compile, I simplified my confusion down to this simple issue.
I had thought that a: => A was a zero argument function returning A just like a: () => A or a: Function0[A]. Apparently, this is not the case at all, and the root of my confusion.
Curiously, I can't declare a local var/val of type => A, I can only use that type for function arguments.
In hindsight, I would guess that this a lazy evaluation notation, not an actual function? Is this true? That is really confusing to use the function arrow for a non-function.
// Compiler Error: Expression of type A doesn't conform to expected type () => A
def notFunction0_1[A](a: => A) { val b: Function0[A] = a }
// Compiler Error: Expression of type A doesn't conform to expected type () => A
def notFunction0_2[A](a: => A) { val b: () => A = a }
// Compiles. Also this matches inferred type of simply A.
def justPlainA[A](a: => A) { val b: A = a }
// While I can declare function arguments of type `=> A`, I can't declare local variables of the same type.
def noLocal[A](a: => A) { val b: => A = a }
