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

Using `atPrec(GlobalPrec)` ensures that `argText(args.head)` never adds
parentheses around its output, but that's inappropriate for a function type
`T1 => T2`. The nested calls to `atPrec` make no sense, but `argText` hid that
before it was refactored.

Author: Blaisorblade

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

@@ -136,7 +136,7 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
       changePrec(GlobalPrec) {
         val argStr: Text =
           if (args.length == 2 && !defn.isTupleType(args.head))
-            atPrec(InfixPrec) { atPrec(GlobalPrec) { argText(args.head) } }
+            atPrec(InfixPrec) { argText(args.head) }
           else
             toTextTuple(args.init)
         (keywordText("erased ") provided isErased) ~ (keywordText("implicit ") provided isImplicit) ~ argStr ~ " => " ~ argText(args.last)

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>