SI-4700 Types with symbolic names print in infix by default

This a partial port of scala/scala#5589 to
dotty: when pretty-printing an applied type, if its type constructor has
a symbolic name, then always print it in infix form. The PR in scalac
also adds an `@showAsInfix` annotation to control this behavior, but we
cannot do the same in dotty since we still rely on the standard library
from Scala 2.11 and the annotation only exists in 2.12 and up.

Author: smarter

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

@@ -124,6 +124,28 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
             toTextTuple(args.init)
         ("implicit " provided isImplicit) ~ argStr ~ " => " ~ argText(args.last)
       }
+
+    def isInfixType(tp: Type): Boolean = tp match {
+      case AppliedType(tycon, args) =>
+        args.length == 2 &&
+          !Character.isUnicodeIdentifierStart(tycon.typeSymbol.name.toString.head)
+          // TODO: Once we use the 2.12 stdlib, also check the @showAsInfix annotation
+      case _ =>
+        false
+    }
+    def toTextInfixType(op: Type, args: List[Type]): Text = {
+      /* SLS 3.2.8: all infix types have the same precedence.
+       * In A op B op' C, op and op' need the same associativity.
+       * Therefore, if op is left associative, anything on its right
+       * needs to be parenthesized if it's an infix type, and vice versa. */
+      val l :: r :: Nil = args
+      val isRightAssoc = op.typeSymbol.name.endsWith(":")
+      val leftArg = if (isRightAssoc && isInfixType(l)) "(" ~ toText(l) ~ ")" else toText(l)
+      val rightArg = if (!isRightAssoc && isInfixType(r)) "(" ~ toText(r) ~ ")" else toText(r)
+
+      leftArg ~ " " ~ toTextLocal(op) ~ " " ~ rightArg
+    }
+
     homogenize(tp) match {
       case x: ConstantType if homogenizedView =>
         return toText(x.widen)
@@ -132,6 +154,7 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
         if (tycon.isRepeatedParam) return toTextLocal(args.head) ~ "*"
         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 tp: TypeRef =>
         val hideType = !ctx.settings.debugAlias.value && (tp.symbol.isAliasPreferred)

tests/repl/infix-types.check

@@ -0,0 +1,19 @@
+scala> class &&[T,U]
+defined class &&
+scala> def foo: Int && Boolean = ???
+def foo: Int && Boolean
+scala> def foo: Int && Boolean && String = ???
+def foo: Int && Boolean && String
+scala> def foo: Int && (Boolean && String) = ???
+def foo: Int && (Boolean && String)
+scala> class &:[L, R]
+defined class &:
+scala> def foo: Int &: String = ???
+def foo: Int &: String
+scala> def foo: Int &: Boolean &: String = ???
+def foo: Int &: Boolean &: String
+scala> def foo: (Int && String) &: Boolean = ???
+def foo: (Int && String) &: Boolean
+scala> def foo: Int && (Boolean &: String) = ???
+def foo: Int && (Boolean &: String)
+scala> :quit