When is a tuple not a tuple, and other scala type error ambiguities

HigherOrderVsCurried.txt

scala> def k( j : (Int => Int) => Int ) : Int = j(_+1)
k: (j: Int => Int => Int)Int

scala> def p(q : Int => Int) : Int = q(0)
p: (q: Int => Int)Int

scala> def z = (x : Int) => (y : Int) => x + y
z: Int => Int => Int

scala> k(p)
res1: Int = 1

scala> k(z)
<console>:10: error: type mismatch;
 found   : Int => Int => Int
 required: Int => Int => Int
              k(z)
                ^

TuplesVsArguments.txt

scala> def f(x : Int, y : Int) : Int = x + y
f: (x: Int, y: Int)Int

scala> def g(z : (Int, Int)) : Int = z._1 + z._2
g: (z: (Int, Int))Int

scala> def h(c : (Int, Int) => Int) : Int = c(1,2)
h: (c: (Int, Int) => Int)Int

scala> f(1,2)
res0: Int = 3

scala> g(1,2)
res1: Int = 3

scala> h(f)
res2: Int = 8

scala> h(g)
<console>:10: error: type mismatch;
 found   : (Int, Int) => Int
 required: (Int, Int) => Int
              h(g)
                ^