Type pretty-printer omits parentheses around by-name params, writing => A => B instead of (=> A) => B

[scabug]

Variation of #5661:

$ ./qbin/scala
Welcome to Scala version 2.10.0-20120925-012651-d6a933c4f5 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_35).
Type in expressions to have them evaluated.
Type :help for more information.

scala> def f(x: => Int): Int = x
f: (x: => Int)Int

scala> f _
res0: => Int => Int = <function1>

Note: d6a933c4f5 is a local commit on top of ffaa3cb89edc1407c99c89f36248358c16887fe7 from 2.10.x, but it's just a small change:

--- a/src/library/scala/package.scala
+++ b/src/library/scala/package.scala
@@ -95,7 +95,10 @@ package object scala {
   val Equiv = scala.math.Equiv
 
   type Fractional[T] = scala.math.Fractional[T]
+  val Fractional = scala.math.Fractional
+
   type Integral[T] = scala.math.Integral[T]
+  val Integral = scala.math.Integral
 
   type Numeric[T] = scala.math.Numeric[T]
   val Numeric = scala.math.Numeric

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-6434?orig=1
Reporter: @Blaisorblade
Assignee: @JamesIry
Affected Versions: 2.9.2, 2.10.0

[scabug]

@jsuereth said:
while the () is useful, I don't think this is actually a bug, more an improvement. IIRC, => A => B => C is a valid function type.

[scabug]

@Blaisorblade said (edited on Sep 30, 2012 6:03:34 PM UTC):
=> A => B is a valid type, just the wrong one. Right-associativity makes that equal to => (A => B) (I'd file a separate bug otherwise), while we want a different type, (=> A) => B; hence omitting parentheses changes the type. The REPL output below suggests that => A => B is indeed right-associative as expected:

scala> val v: ((=> Int) => Int) = f _
v: => Int => Int = <function1>

scala> val v: (=> (Int => Int)) = f _
<console>:1: error: no by-name parameter type allowed here
       val v: (=> (Int => Int)) = f _
               ^

scala> val v: (=> Int => Int) = f _
<console>:1: error: no by-name parameter type allowed here
       val v: (=> Int => Int) = f _
               ^

[scabug]

@adriaanm said:
James, care to put some more parens in our types? (Because we like parens.)

[scabug]

@JamesIry said:
scala/scala#1958