Fix infinite recursion with -Yplain-printer

Trying to print the type of `List(1,2)` in the repl launched with
`-Yplain-printer` previously resulted in an infinite recursion (stopped
by PlainPrinter#controlled) because `refinementChain(tp)` will return a
list of only one argument for a RefinedType which is also an
AppliedType.

Author: smarter

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

@@ -142,6 +142,8 @@ class PlainPrinter(_ctx: Context) extends Printer {
         toTextLocal(tp.underlying) ~ "(" ~ toTextRef(tp) ~ ")"
       case tp: TypeRef =>
         toTextPrefix(tp.prefix) ~ selectionString(tp)
+      case AppliedType(tycon, args) =>
+        (toTextLocal(tycon) ~ "[" ~ Text(args map argText, ", ") ~ "]").close
       case tp: RefinedType =>
         val parent :: (refined: List[RefinedType @unchecked]) =
           refinementChain(tp).reverse

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

@@ -155,7 +155,6 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
         if (defn.isFunctionClass(cls)) return toTextFunction(args, cls.name.isImplicitFunction)
         if (defn.isTupleClass(cls)) return toTextTuple(args)
         if (isInfixType(tp)) return toTextInfixType(tycon, args)
-        return (toTextLocal(tycon) ~ "[" ~ Text(args map argText, ", ") ~ "]").close
       case EtaExpansion(tycon) =>
         return toText(tycon)
       case tp: TypeRef =>