why is this cast required?

[DanielG]

I see, thank you!

Do you (or anyone else) know the reason for that limitation? Is 
there some obvious situation where the ambiguity is preferable, 
versus simply inferring - or trying to infer - the expected type?

ie, superficially it seems like the compiler could say, "Hey, 
they are trying to use this array in the context of a [type X] 
argument - let's assume that for the moment, and see if it works."

Put another way, why is a function argument treated differently 
than the right side of an explicitly-typed variable assignment?