Fix #4068: Fix pretty-printing precedence for function types

Fix #4068 in a hacky way.

Author: Blaisorblade

compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala

@@ -135,7 +135,9 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
     def toTextFunction(args: List[Type], isImplicit: Boolean, isErased: Boolean): Text =
       changePrec(GlobalPrec) {
         val argStr: Text =
-          if (args.length == 2 && !defn.isTupleType(args.head))
+          if (args.length == 2 && defn.isFunctionType(args.head)) {
+            "(" ~ globalPrecArgText(args.head) ~ ")"
+          } else if (args.length == 2 && !defn.isTupleType(args.head))
             atPrec(InfixPrec) { globalPrecArgText(args.head) }
           else
             toTextTuple(args.init)

compiler/test-resources/type-printer/functions

@@ -1,2 +1,10 @@
 scala> val toInt: Any => Int = new { def apply(a: Any) = 1; override def toString() = "<func1>" }
 val toInt: Any => Int = <func1>
+scala> val hoFun: (Int => Int) => Int = new { def apply(a: Int => Int) = 1; override def toString() = "<func2>" }
+val hoFun: (Int => Int) => Int = <func2>
+scala> val curriedFun: Int => (Int => Int) = new { def apply(a: Int) = _ => 1; override def toString() = "<func3>" }
+val curriedFun: Int => Int => Int = <func3>
+scala> val tupFun: ((Int, Int)) => Int = new { def apply(a: (Int, Int)) = 1; override def toString() = "<func4>" }
+val tupFun: ((Int, Int)) => Int = <func4>
+scala> val binFun: (Int, Int) => Int = new { def apply(a: Int, b: Int) = 1; override def toString() = "<func5>" }
+val binFun: (Int, Int) => Int = <func5>