Implement structural type member access

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -629,6 +629,28 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
       case nil =>
         Nil
     }
+
+  /** Is this a selection of a member of a structural type that is not a member
+   *  of an underlying class or trait?
+   */
+  def isStructuralTermSelect(tree: Tree)(implicit ctx: Context) = tree match {
+    case tree: Select =>
+      def hasRefinement(qualtpe: Type): Boolean = qualtpe.dealias match {
+        case RefinedType(parent, rname, rinfo) =>
+          rname == tree.name || hasRefinement(parent)
+        case tp: TypeProxy =>
+          hasRefinement(tp.underlying)
+        case tp: OrType =>
+          hasRefinement(tp.tp1) || hasRefinement(tp.tp2)
+        case tp: AndType =>
+          hasRefinement(tp.tp1) && hasRefinement(tp.tp2)
+        case _ =>
+          false
+      }
+      !tree.symbol.exists && tree.isTerm && hasRefinement(tree.qualifier.tpe)
+    case _ =>
+      false
+  }
 }
 
 object TreeInfo {

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -26,6 +26,9 @@ object Definitions {
    *  else without affecting the set of programs that can be compiled.
    */
   val MaxImplementedFunctionArity = 22
+
+  /** The maximal arity of a function that can be accessed as member of a structural type */
+  val MaxStructuralMethodArity = 7
 }
 
 /** A class defining symbols and types of standard definitions
@@ -505,6 +508,7 @@ class Definitions {
   lazy val LanguageModuleRef = ctx.requiredModule("scala.language")
   def LanguageModuleClass(implicit ctx: Context) = LanguageModuleRef.symbol.moduleClass.asClass
   lazy val NonLocalReturnControlType: TypeRef   = ctx.requiredClassRef("scala.runtime.NonLocalReturnControl")
+  lazy val SelectableType: TypeRef              = ctx.requiredClassRef("scala.Selectable")
 
   lazy val ClassTagType = ctx.requiredClassRef("scala.reflect.ClassTag")
   def ClassTagClass(implicit ctx: Context) = ClassTagType.symbol.asClass

compiler/src/dotty/tools/dotc/core/StdNames.scala

@@ -481,6 +481,7 @@ object StdNames {
     val sameElements: N         = "sameElements"
     val scala_ : N              = "scala"
     val selectDynamic: N        = "selectDynamic"
+    val selectDynamicMethod: N  = "selectDynamicMethod"
     val selectOverloadedMethod: N = "selectOverloadedMethod"
     val selectTerm: N           = "selectTerm"
     val selectType: N           = "selectType"

compiler/src/dotty/tools/dotc/typer/Dynamic.scala

@@ -7,25 +7,35 @@ import dotty.tools.dotc.ast.tpd
 import dotty.tools.dotc.ast.untpd
 import dotty.tools.dotc.core.Constants.Constant
 import dotty.tools.dotc.core.Contexts.Context
-import dotty.tools.dotc.core.Names.Name
+import dotty.tools.dotc.core.Names.{Name, TermName}
 import dotty.tools.dotc.core.StdNames._
 import dotty.tools.dotc.core.Types._
 import dotty.tools.dotc.core.Decorators._
+import core.Symbols._
+import core.Definitions
+import Inferencing._
 import ErrorReporting._
 
 object Dynamic {
   def isDynamicMethod(name: Name): Boolean =
     name == nme.applyDynamic || name == nme.selectDynamic || name == nme.updateDynamic || name == nme.applyDynamicNamed
 }
 
-/** Translates selection that does not typecheck according to the scala.Dynamic rules:
+/** Handles programmable member selections of `Dynamic` instances and values
+ *  with structural types. Two functionalities:
+ *
+ * 1. Translates selection that does not typecheck according to the scala.Dynamic rules:
  *    foo.bar(baz) = quux                   ~~> foo.selectDynamic(bar).update(baz, quux)
  *    foo.bar = baz                         ~~> foo.updateDynamic("bar")(baz)
  *    foo.bar(x = bazX, y = bazY, baz, ...) ~~> foo.applyDynamicNamed("bar")(("x", bazX), ("y", bazY), ("", baz), ...)
  *    foo.bar(baz0, baz1, ...)              ~~> foo.applyDynamic(bar)(baz0, baz1, ...)
  *    foo.bar                               ~~> foo.selectDynamic(bar)
  *
  *  The first matching rule of is applied.
+ *
+ * 2. Translates member selections on structural types to calls of `selectDynamic`
+ *    or `selectDynamicMethod` on a `Selectable` instance. @See handleStructural.
+ *
  */
 trait Dynamic { self: Typer with Applications =>
   import Dynamic._
@@ -100,4 +110,56 @@ trait Dynamic { self: Typer with Applications =>
       else untpd.TypeApply(select, targs)
     untpd.Apply(selectWithTypes, Literal(Constant(name.toString)))
   }
+
+  /** Handle reflection-based dispatch for members of structural types.
+   *  Given `x.a`, where `x` is of (widened) type `T` and `x.a` is of type `U`:
+   *
+   *  If `U` is a value type, map `x.a` to the equivalent of:
+   *
+   *     (x: Selectable).selectDynamic(x, "a").asInstanceOf[U]
+   *
+   *  If `U` is a method type (T1,...,Tn)R, map `x.a` to the equivalent of:
+   *
+   *     (x: Selectable).selectDynamicMethod("a", CT1, ..., CTn).asInstanceOf[(T1,...,Tn) => R]
+   *
+   *  where CT1,...,CTn are the class tags representing the erasure of T1,...,Tn.
+   *
+   *  It's an error if U is neither a value nor a method type, or a dependent method
+   *  type, or of too large arity (limit is Definitions.MaxStructuralMethodArity).
+   */
+  def handleStructural(tree: Tree)(implicit ctx: Context): Tree = {
+    val Select(qual, name) = tree
+
+    def structuralCall(selectorName: TermName, formals: List[Tree]) = {
+      val selectable = adapt(qual, defn.SelectableType)
+      val scall = untpd.Apply(
+        untpd.TypedSplice(selectable.select(selectorName)),
+        (Literal(Constant(name.toString)) :: formals).map(untpd.TypedSplice(_)))
+      typed(scall)
+    }
+
+    def fail(reason: String) =
+      errorTree(tree, em"Structural access not allowed on method $name because it $reason")
+
+    tree.tpe.widen match {
+      case tpe: MethodType =>
+        if (tpe.isDependent)
+          fail(i"has a dependent method type")
+        else if (tpe.paramNames.length > Definitions.MaxStructuralMethodArity)
+          fail(i"""takes too many parameters.
+                  |Structural types only support methods taking up to ${Definitions.MaxStructuralMethodArity} arguments""")
+        else {
+          def issueError(msgFn: String => String): Unit = ctx.error(msgFn(""), tree.pos)
+          val ctags = tpe.paramTypes.map(pt =>
+            inferImplicitArg(defn.ClassTagType.appliedTo(pt :: Nil), issueError, tree.pos.endPos))
+          structuralCall(nme.selectDynamicMethod, ctags).asInstance(tpe.toFunctionType())
+        }
+      case tpe: ValueType =>
+        structuralCall(nme.selectDynamic, Nil).asInstance(tpe)
+      case tpe: PolyType =>
+        fail("is polymorphic")
+      case tpe =>
+        fail(i"has an unsupported type: $tpe")
+    }
+  }
 }

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1040,9 +1040,8 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
       typr.println(s"adding refinement $refinement")
       checkRefinementNonCyclic(refinement, refineCls, seen)
       val rsym = refinement.symbol
-      if (rsym.is(Method) && rsym.allOverriddenSymbols.isEmpty)
-        ctx.error(i"refinement $rsym without matching type in parent $tpt1", refinement.pos)
-    }
+      if (rsym.info.isInstanceOf[PolyType] && rsym.allOverriddenSymbols.isEmpty)
+        ctx.error(i"polymorphic refinement $rsym without matching type in parent $tpt1 is no longer allowed", refinement.pos)    }
     assignType(cpy.RefinedTypeTree(tree)(tpt1, refinements1), tpt1, refinements1, refineCls)
   }
 
@@ -2045,7 +2044,8 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
               adaptInterpolated(tree.appliedToTypeTrees(typeArgs), pt, original))
           }
         case wtp =>
-          pt match {
+          if (isStructuralTermSelect(tree)) adapt(handleStructural(tree), pt)
+          else pt match {
             case pt: FunProto =>
               adaptToArgs(wtp, pt)
             case pt: PolyProto =>

library/src/scala/Selectable.scala

@@ -0,0 +1,8 @@
+package scala
+import scala.reflect.ClassTag
+
+trait Selectable extends Any {
+  def selectDynamic(name: String): Any
+  def selectDynamicMethod(name: String, paramClasses: ClassTag[_]*): Any =
+    new UnsupportedOperationException("selectDynamicMethod")
+}

library/src/scala/reflect/Selectable.scala

@@ -0,0 +1,73 @@
+package scala.reflect
+
+class Selectable(val receiver: Any) extends AnyVal with scala.Selectable {
+  def selectDynamic(name: String): Any = {
+    val rcls = receiver.getClass
+    try {
+      val fld = rcls.getField(name)
+      fld.get(receiver)
+    }
+    catch {
+      case ex: NoSuchFieldError =>
+        selectDynamicMethod(name).asInstanceOf[() => Any]()
+    }
+  }
+
+  override def selectDynamicMethod(name: String, paramTypes: ClassTag[_]*): Any = {
+    val rcls = receiver.getClass
+    val paramClasses = paramTypes.map(_.runtimeClass)
+    val mth = rcls.getMethod(name, paramClasses: _*)
+    paramTypes.length match {
+      case 0 => () =>
+        mth.invoke(receiver)
+      case 1 => (x0: Any) =>
+        mth.invoke(receiver, x0.asInstanceOf[Object])
+      case 2 => (x0: Any, x1: Any) =>
+        mth.invoke(receiver,
+            x0.asInstanceOf[Object],
+            x1.asInstanceOf[Object])
+      case 3 => (x0: Any, x1: Any, x2: Any) =>
+        mth.invoke(receiver,
+            x0.asInstanceOf[Object],
+            x1.asInstanceOf[Object],
+            x2.asInstanceOf[Object])
+      case 4 => (x0: Any, x1: Any, x2: Any, x3: Any) =>
+        mth.invoke(receiver,
+            x0.asInstanceOf[Object],
+            x1.asInstanceOf[Object],
+            x2.asInstanceOf[Object],
+            x3.asInstanceOf[Object])
+      case 5 => (x0: Any, x1: Any, x2: Any, x3: Any, x4: Any) =>
+        mth.invoke(receiver,
+            x0.asInstanceOf[Object],
+            x1.asInstanceOf[Object],
+            x2.asInstanceOf[Object],
+            x3.asInstanceOf[Object],
+            x4.asInstanceOf[Object])
+      case 6 => (x0: Any, x1: Any, x2: Any, x3: Any, x4: Any, x5: Any) =>
+        mth.invoke(receiver,
+            x0.asInstanceOf[Object],
+            x1.asInstanceOf[Object],
+            x2.asInstanceOf[Object],
+            x3.asInstanceOf[Object],
+            x4.asInstanceOf[Object],
+            x5.asInstanceOf[Object])
+      case 7 => (x0: Any, x1: Any, x2: Any, x3: Any, x4: Any, x5: Any, x6: Any) =>
+        mth.invoke(receiver,
+            x0.asInstanceOf[Object],
+            x1.asInstanceOf[Object],
+            x2.asInstanceOf[Object],
+            x3.asInstanceOf[Object],
+            x4.asInstanceOf[Object],
+            x5.asInstanceOf[Object],
+            x6.asInstanceOf[Object])
+      }
+  }
+}
+
+object Selectable {
+  implicit def reflectiveSelectable(receiver: Any): scala.Selectable = receiver match {
+    case receiver: scala.Selectable => receiver
+    case _                          => new Selectable(receiver)
+  }
+}

tests/neg/structural.scala

@@ -0,0 +1,11 @@
+object Test3 {
+  import scala.reflect.Selectable.reflectiveSelectable
+  def g(x: { type T ; def t: T ; def f(a: T): Boolean }) = x.f(x.t) // error: no ClassTag for x.T
+  g(new { type T = Int; def t = 4; def f(a:T) = true })
+  g(new { type T = Any; def t = 4; def f(a:T) = true })
+  val y: { type T = Int; def t = 4; def f(a:T) = true }
+    = new { type T = Int; def t = 4; def f(a:T) = true }
+
+  def h(x: { def f[T](a: T): Int }) = x.f[Int](4) // error: polymorphic refinement method ... no longer allowed
+
+}

tests/neg/zoo.scala

@@ -7,19 +7,19 @@ type Grass = {
 }
 type Animal = {
   type Food
-  def eats(food: Food): Unit   // error
-  def gets: Food               // error
+  def eats(food: Food): Unit
+  def gets: Food
 }
 type Cow = {
   type IsMeat = Any
   type Food <: Grass
-  def eats(food: Grass): Unit  // error
-  def gets: Grass              // error
+  def eats(food: Grass): Unit
+  def gets: Grass
 }
 type Lion = {
   type Food = Meat
-  def eats(food: Meat): Unit   // error
-  def gets: Meat               // error
+  def eats(food: Meat): Unit
+  def gets: Meat
 }
 def newMeat: Meat = new {
   type IsMeat = Any
@@ -40,5 +40,5 @@ def newLion: Lion = new {
 }
 val milka = newCow
 val leo = newLion
-leo.eats(milka) // structural select not supported
+leo.eats(milka)  // error: no projector found
 }

tests/pos/i1866.scala

@@ -0,0 +1,5 @@
+import scala.reflect.Selectable.reflectiveSelectable
+object Test {
+  def f(g: { val update: Unit }) = g.update
+  def main(update: Array[String]) = {}
+}

tests/pos/zoo2.scala

@@ -0,0 +1,45 @@
+import scala.reflect.Selectable.reflectiveSelectable
+object Test {
+type Meat = {
+  type IsMeat = Any
+}
+type Grass = {
+  type IsGrass = Any
+}
+type Animal = {
+  type Food
+  def eats(food: Food): Unit
+  def gets: Food
+}
+type Cow = {
+  type IsMeat = Any
+  type Food <: Grass
+  def eats(food: Grass): Unit
+  def gets: Grass
+}
+type Lion = {
+  type Food = Meat
+  def eats(food: Meat): Unit
+  def gets: Meat
+}
+def newMeat: Meat = new {
+  type IsMeat = Any
+}
+def newGrass: Grass = new {
+  type IsGrass = Any
+}
+def newCow: Cow = new {
+  type IsMeat = Any
+  type Food = Grass
+  def eats(food: Grass) = ()
+  def gets = newGrass
+}
+def newLion: Lion = new {
+  type Food = Meat
+  def eats(food: Meat) = ()
+  def gets = newMeat
+}
+val milka = newCow
+val leo = newLion
+leo.eats(milka)
+}

tests/run/structural.scala

@@ -0,0 +1,25 @@
+case class Record(elems: (String, Any)*) extends Selectable {
+  def selectDynamic(name: String): Any = elems.find(_._1 == name).get._2
+}
+
+object Test {
+  import scala.reflect.Selectable.reflectiveSelectable
+
+  def f(closeable: { def close(): Unit }) =
+    closeable.close()
+
+  type RN = Record { val name: String; val age: Int }
+
+  def g(r: RN) = r.name
+
+  val rr: RN = Record("name" -> "Bob", "age" -> 42).asInstanceOf[RN]
+
+  def main(args: Array[String]): Unit = {
+    f(new java.io.PrintStream("foo"))
+    assert(g(rr) == "Bob")
+
+    val s: { def concat(s: String): String } = "abc"
+    assert(s.concat("def") == "abcdef")
+  }
+}
+

tests/run/structuralNoSuchMethod.check

@@ -0,0 +1 @@
+no such method