scala · adriaanm · Feb 16, 2017 · Sep 14, 2016 · Dec 8, 2016 · Dec 8, 2016
diff --git a/src/library/scala/annotation/showAsInfix.scala b/src/library/scala/annotation/showAsInfix.scala
@@ -0,0 +1,27 @@
+package scala.annotation
+
+/**
+ * This annotation configures how Scala prints two-parameter generic types.
+  *
+  * By default, types with symbolic names are printed infix; while types without
+  * them are printed using the regular generic type syntax.
+  *
+  * Example of usage:
+  {{{
+    scala> class Map[T, U]
+    defined class Map
+
+    scala> def foo: Int Map Int = ???
+    foo: Map[Int,Int]
+
+    scala> @showAsInfix class Map[T, U]
+    defined class Map
+
+    scala> def foo: Int Map Int = ???
+    foo: Int Map Int
+  }}}
+  *
+  * @param enabled whether to show this type as an infix type operator.
+  * @since 2.12.2
+  */
+class showAsInfix(enabled: Boolean = true) extends annotation.StaticAnnotation
diff --git a/src/reflect/scala/reflect/internal/AnnotationInfos.scala b/src/reflect/scala/reflect/internal/AnnotationInfos.scala
@@ -316,8 +316,9 @@ trait AnnotationInfos extends api.Annotations { self: SymbolTable =>
     /** Check whether any of the arguments mention a symbol */
     def refsSymbol(sym: Symbol) = hasArgWhich(_.symbol == sym)
 
-    def stringArg(index: Int) = constantAtIndex(index) map (_.stringValue)
-    def intArg(index: Int)    = constantAtIndex(index) map (_.intValue)
+    def stringArg(index: Int)  = constantAtIndex(index) map (_.stringValue)
+    def intArg(index: Int)     = constantAtIndex(index) map (_.intValue)
+    def booleanArg(index: Int) = constantAtIndex(index) map (_.booleanValue)
     def symbolArg(index: Int) = argAtIndex(index) collect {
       case Apply(fun, Literal(str) :: Nil) if fun.symbol == definitions.Symbol_apply =>
         newTermName(str.stringValue)

diff --git a/src/reflect/scala/reflect/internal/Definitions.scala b/src/reflect/scala/reflect/internal/Definitions.scala
@@ -1405,6 +1405,8 @@ trait Definitions extends api.StandardDefinitions {
       case _                  => false
     }
 
+    lazy val ShowAsInfixAnnotationClass = rootMirror.getClassIfDefined("scala.annotation.showAsInfix")
+
     // todo: reconcile with javaSignature!!!
     def signature(tp: Type): String = {
       def erasure(tp: Type): Type = tp match {

diff --git a/src/reflect/scala/reflect/internal/Types.scala b/src/reflect/scala/reflect/internal/Types.scala
@@ -307,6 +307,9 @@ trait Types
     /** Is this type completed (i.e. not a lazy type)? */
     def isComplete: Boolean = true
 
+    /** Should this be printed as an infix type (@showAsInfix class &&[T, U])? */
+    def isShowAsInfixType: Boolean = false
+
     /** If this is a lazy type, assign a new type to `sym`. */
     def complete(sym: Symbol) {}
 
@@ -2097,6 +2100,15 @@ trait Types
         trivial = fromBoolean(!sym.isTypeParameter && pre.isTrivial && areTrivialTypes(args))
       toBoolean(trivial)
     }
+
+    /* It only makes sense to show 2-ary type constructors infix.
+     * By default we do only if it's a symbolic name. */
+    override def isShowAsInfixType: Boolean =
+      hasLength(args, 2) &&
+        sym.getAnnotation(ShowAsInfixAnnotationClass)
+         .map(_ booleanArg 0 getOrElse true)
+         .getOrElse(!Character.isUnicodeIdentifierStart(sym.decodedName.head))
+
     private[Types] def invalidateTypeRefCaches(): Unit = {
       parentsCache = null
       parentsPeriod = NoPeriod
@@ -2322,6 +2334,22 @@ trait Types
       case arg :: Nil => s"($arg,)"
       case _          => args.mkString("(", ", ", ")")
     }
+    private def infixTypeString: String = {
+      /* SLS 3.2.8: all infix types have the same precedence.
+       * In A op B op' C, op and op' need the same associativity.
+       * Therefore, if op is left associative, anything on its right
+       * needs to be parenthesized if it's an infix type, and vice versa. */
+      // we should only get here after `isShowInfixType` says we have 2 args
+      val l :: r :: Nil = args
+
+      val isRightAssoc = typeSymbol.decodedName endsWith ":"
+
+      val lstr = if (isRightAssoc && l.isShowAsInfixType) s"($l)" else l.toString
+
+      val rstr = if (!isRightAssoc && r.isShowAsInfixType) s"($r)" else r.toString
+
+      s"$lstr ${sym.decodedName} $rstr"
+    }
     private def customToString = sym match {
       case RepeatedParamClass | JavaRepeatedParamClass => args.head + "*"
       case ByNameParamClass   => "=> " + args.head
@@ -2345,6 +2373,8 @@ trait Types
               xs.init.mkString("(", ", ", ")") + " => " + xs.last
           }
         }
+        else if (isShowAsInfixType)
+          infixTypeString
         else if (isTupleTypeDirect(this))
           tupleTypeString
         else if (sym.isAliasType && prefixChain.exists(_.termSymbol.isSynthetic) && (this ne dealias))

diff --git a/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala b/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala
@@ -425,6 +425,7 @@ trait JavaUniverseForce { self: runtime.JavaUniverse  =>
     definitions.hijackedCoreClasses
     definitions.symbolsNotPresentInBytecode
     definitions.isPossibleSyntheticParent
+    definitions.ShowAsInfixAnnotationClass
     definitions.abbrvTag
     definitions.numericWeight
     definitions.boxedModule

diff --git a/test/files/run/t4700.check b/test/files/run/t4700.check
@@ -0,0 +1,44 @@
+
+scala> import scala.annotation.showAsInfix
+import scala.annotation.showAsInfix
+
+scala> class &&[T,U]
+defined class $amp$amp
+
+scala> def foo: Int && Boolean = ???
+foo: Int && Boolean
+
+scala> def foo: Int && Boolean && String = ???
+foo: Int && Boolean && String
+
+scala> def foo: Int && (Boolean && String) = ???
+foo: Int && (Boolean && String)
+
+scala> @showAsInfix type Mappy[T, U] = Map[T, U]
+defined type alias Mappy
+
+scala> def foo: Int Mappy (Boolean && String) = ???
+foo: Int Mappy (Boolean && String)
+
+scala> @showAsInfix(false) class ||[T,U]
+defined class $bar$bar
+
+scala> def foo: Int || Boolean = ???
+foo: ||[Int,Boolean]
+
+scala> class &:[L, R]
+defined class $amp$colon
+
+scala> def foo: Int &: String = ???
+foo: Int &: String
+
+scala> def foo: Int &: Boolean &: String = ???
+foo: Int &: Boolean &: String
+
+scala> def foo: (Int && String) &: Boolean = ???
+foo: (Int && String) &: Boolean
+
+scala> def foo: Int && (Boolean &: String) = ???
+foo: Int && (Boolean &: String)
+
+scala> :quit
diff --git a/test/files/run/t4700.scala b/test/files/run/t4700.scala
@@ -0,0 +1,22 @@
+import scala.tools.nsc.interpreter._
+import scala.tools.partest.ReplTest
+
+object Test extends ReplTest {
+  def code = """
+    |import scala.annotation.showAsInfix
+    |class &&[T,U]
+    |def foo: Int && Boolean = ???
+    |def foo: Int && Boolean && String = ???
+    |def foo: Int && (Boolean && String) = ???
+    |@showAsInfix type Mappy[T, U] = Map[T, U]
+    |def foo: Int Mappy (Boolean && String) = ???
+    |@showAsInfix(false) class ||[T,U]
+    |def foo: Int || Boolean = ???
+    |class &:[L, R]
+    |def foo: Int &: String = ???
+    |def foo: Int &: Boolean &: String = ???
+    |def foo: (Int && String) &: Boolean = ???
+    |def foo: Int && (Boolean &: String) = ???
+    |""".stripMargin
+}
+