Implement simple phantom functions

Simple phantom functions are FunctionN/ImplicitFunctionN types
where all parameters are of the same phantom universe. They are
synthesized in the object defining the phantom universe. It main
usage should be through `(MyPhantom) => Int` nothation as it does
not require any imports.

Author: nicolasstucki

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

@@ -60,14 +60,14 @@ class Definitions {
   private def enterCompleteClassSymbol(owner: Symbol, name: TypeName, flags: FlagSet, parents: List[TypeRef], decls: Scope = newScope) =
     ctx.newCompleteClassSymbol(owner, name, flags | Permanent, parents, decls).entered
 
-  private def enterTypeField(cls: ClassSymbol, name: TypeName, flags: FlagSet, scope: MutableScope) =
-    scope.enter(newSymbol(cls, name, flags, TypeBounds.empty))
+  private def enterTypeField(cls: ClassSymbol, name: TypeName, flags: FlagSet, scope: MutableScope, typeBounds: TypeBounds) =
+    scope.enter(newSymbol(cls, name, flags, typeBounds))
 
-  private def enterTypeParam(cls: ClassSymbol, name: TypeName, flags: FlagSet, scope: MutableScope) =
-    enterTypeField(cls, name, flags | ClassTypeParamCreationFlags, scope)
+  private def enterTypeParam(cls: ClassSymbol, name: TypeName, flags: FlagSet, scope: MutableScope, typeBounds: TypeBounds) =
+    enterTypeField(cls, name, flags | ClassTypeParamCreationFlags, scope, typeBounds)
 
   private def enterSyntheticTypeParam(cls: ClassSymbol, paramFlags: FlagSet, scope: MutableScope, suffix: String = "T0") =
-    enterTypeParam(cls, suffix.toTypeName.expandedName(cls), paramFlags, scope)
+    enterTypeParam(cls, suffix.toTypeName.expandedName(cls), paramFlags, scope, TypeBounds.empty)
 
   // NOTE: Ideally we would write `parentConstrs: => Type*` but SIP-24 is only
   // implemented in Dotty and not in Scala 2.
@@ -108,33 +108,31 @@ class Definitions {
    *        def apply(implicit $x0: T0, ..., $x{N_1}: T{N-1}): R
    *      }
    */
-  def newFunctionNTrait(name: TypeName): ClassSymbol = {
+  def newFunctionNTrait(name: TypeName, lattice: Symbol): ClassSymbol = {
     val completer = new LazyType {
       def complete(denot: SymDenotation)(implicit ctx: Context): Unit = {
         val cls = denot.asClass.classSymbol
         val decls = newScope
         val arity = name.functionArity
+        val top = lattice.thisType.select(tpnme.Any)
+        val bottom = lattice.thisType.select(tpnme.Nothing)
         val paramNamePrefix = tpnme.scala_ ++ str.NAME_JOIN ++ name ++ str.EXPAND_SEPARATOR
         val argParams =
           for (i <- List.range(1, arity + 1)) yield
-            enterTypeParam(cls, paramNamePrefix ++ "T" ++ i.toString, Contravariant, decls)
-        val resParam = enterTypeParam(cls, paramNamePrefix ++ "R", Covariant, decls)
+            enterTypeParam(cls, paramNamePrefix ++ "T" ++ i.toString, Contravariant, decls, TypeBounds(bottom, top)).typeRef
+        val resParam = enterTypeParam(cls, paramNamePrefix ++ "R", Covariant, decls, TypeBounds.empty).typeRef
         val (methodType, parentTraits) =
           if (name.firstPart.startsWith(str.ImplicitFunction)) {
             val superTrait =
-              FunctionType(arity).appliedTo(argParams.map(_.typeRef) ::: resParam.typeRef :: Nil)
+              FunctionType(arity, isImplicit = false, top).appliedTo(argParams ::: resParam :: Nil)
             (ImplicitMethodType, ctx.normalizeToClassRefs(superTrait :: Nil, cls, decls))
           }
           else (MethodType, Nil)
-        val applyMeth =
-          decls.enter(
-            newMethod(cls, nme.apply,
-              methodType(argParams.map(_.typeRef), resParam.typeRef), Deferred))
-        denot.info =
-          ClassInfo(ScalaPackageClass.thisType, cls, ObjectType :: parentTraits, decls)
+        decls.enter(newMethod(cls, nme.apply, methodType(argParams, resParam), Deferred))
+        denot.info = ClassInfo(lattice.thisType, cls, ObjectType :: parentTraits, decls)
       }
     }
-    newClassSymbol(ScalaPackageClass, name, Trait | NoInits, completer)
+    newClassSymbol(lattice, name, Trait | NoInits, completer)
   }
 
   private def newMethod(cls: ClassSymbol, name: TermName, info: Type, flags: FlagSet = EmptyFlags): TermSymbol =
@@ -194,7 +192,7 @@ class Definitions {
     val cls = ScalaPackageVal.moduleClass.asClass
     cls.info.decls.openForMutations.useSynthesizer(
       name => ctx =>
-        if (name.isTypeName && name.isSyntheticFunction) newFunctionNTrait(name.asTypeName)
+        if (name.isTypeName && name.isSyntheticScalaFunction) newFunctionNTrait(name.asTypeName, cls)
         else NoSymbol)
     cls
   }
@@ -693,7 +691,7 @@ class Definitions {
 
   object FunctionOf {
     def apply(args: List[Type], resultType: Type, isImplicit: Boolean = false)(implicit ctx: Context) =
-      FunctionType(args.length, isImplicit).appliedTo(args ::: resultType :: Nil)
+      FunctionType(args, resultType, isImplicit).appliedTo(args ::: resultType :: Nil)
     def unapply(ft: Type)(implicit ctx: Context) = {
       val tsym = ft.typeSymbol
       if (isFunctionClass(tsym)) {
@@ -773,16 +771,39 @@ class Definitions {
     lazy val Function0_applyR = ImplementedFunctionType(0).symbol.requiredMethodRef(nme.apply)
     def Function0_apply(implicit ctx: Context) = Function0_applyR.symbol
 
-  def FunctionType(n: Int, isImplicit: Boolean = false)(implicit ctx: Context): TypeRef =
-    if (n <= MaxImplementedFunctionArity && (!isImplicit || ctx.erasedTypes)) ImplementedFunctionType(n)
+  def FunctionType(n: Int, isImplicit: Boolean = false, top: Type = AnyType)(implicit ctx: Context): TypeRef = {
+    if (top.isPhantom) {
+      val functionPrefix = if (isImplicit) str.ImplicitFunction else str.Function
+      val functionName = (functionPrefix + n).toTypeName
+      val functionType = top.normalizedPrefix.select(functionName)
+      assert(functionType.classSymbol.exists)
+      functionType.asInstanceOf[TypeRef]
+    }
+    else if (n <= MaxImplementedFunctionArity && (!isImplicit || ctx.erasedTypes)) ImplementedFunctionType(n)
     else FunctionClass(n, isImplicit).typeRef
+  }
+
+  def FunctionType(args: List[Type], resultType: Type, isImplicit: Boolean)(implicit ctx: Context): TypeRef =
+    FunctionType(args.length, isImplicit, topInSameUniverse(args, "function arguments."))
+
+  private def topInSameUniverse(types: List[Type], relationship: => String)(implicit ctx: Context): Type = {
+    types match {
+      case first :: Nil => first.topType
+      case first :: rest => (first /: rest)(inSameUniverse((t1, _) => t1.topType, _, _, relationship, ctx.owner.pos))
+      case Nil => defn.AnyType
+    }
+  }
 
   private lazy val TupleTypes: Set[TypeRef] = TupleType.toSet
 
   /** If `cls` is a class in the scala package, its name, otherwise EmptyTypeName */
   def scalaClassName(cls: Symbol)(implicit ctx: Context): TypeName =
     if (cls.isClass && cls.owner == ScalaPackageClass) cls.asClass.name else EmptyTypeName
 
+  /** If `cls` is a class in an object extending scala.Phantom, its name, otherwise EmptyTypeName */
+  def phantomClassName(cls: Symbol)(implicit ctx: Context): TypeName =
+    if (cls.isClass && cls.owner.derivesFrom(PhantomClass)) cls.asClass.name else EmptyTypeName
+
   /** If type `ref` refers to a class in the scala package, its name, otherwise EmptyTypeName */
   def scalaClassName(ref: Type)(implicit ctx: Context): TypeName = scalaClassName(ref.classSymbol)
 
@@ -801,24 +822,28 @@ class Definitions {
    *   - FunctionN for N >= 0
    *   - ImplicitFunctionN for N > 0
    */
-  def isFunctionClass(cls: Symbol) = scalaClassName(cls).isFunction
+  def isFunctionClass(cls: Symbol) =
+    scalaClassName(cls).isFunction || phantomClassName(cls).isFunction
 
   /** Is an implicit function class.
    *   - ImplicitFunctionN for N > 0
    */
-  def isImplicitFunctionClass(cls: Symbol) = scalaClassName(cls).isImplicitFunction
+  def isImplicitFunctionClass(cls: Symbol) =
+    scalaClassName(cls).isImplicitFunction || phantomClassName(cls).isImplicitFunction
 
   /** Is a class that will be erased to FunctionXXL
    *   - FunctionN for N >= 22
    *   - ImplicitFunctionN for N >= 22
    */
-  def isXXLFunctionClass(cls: Symbol) = scalaClassName(cls).functionArity > MaxImplementedFunctionArity
+  def isXXLFunctionClass(cls: Symbol) =
+    scalaClassName(cls).functionArity > MaxImplementedFunctionArity
 
   /** Is a synthetic function class
    *    - FunctionN for N > 22
    *    - ImplicitFunctionN for N > 0
    */
-  def isSyntheticFunctionClass(cls: Symbol) = scalaClassName(cls).isSyntheticFunction
+  def isSyntheticFunctionClass(cls: Symbol) =
+    scalaClassName(cls).isSyntheticScalaFunction || phantomClassName(cls).isFunction
 
   def isAbstractFunctionClass(cls: Symbol) = isVarArityClass(cls, str.AbstractFunction)
   def isTupleClass(cls: Symbol) = isVarArityClass(cls, str.Tuple)
@@ -835,6 +860,7 @@ class Definitions {
     val arity = scalaClassName(cls).functionArity
     if (arity > 22) FunctionXXLClass
     else if (arity >= 0) FunctionClass(arity)
+    else if (phantomClassName(cls).isFunction) FunctionClass(0)
     else NoSymbol
   }
 
@@ -847,8 +873,9 @@ class Definitions {
    */
   def erasedFunctionType(cls: Symbol): Type = {
     val arity = scalaClassName(cls).functionArity
-    if (arity > 22) defn.FunctionXXLType
-    else if (arity >= 0) defn.FunctionType(arity)
+    if (arity > 22) FunctionXXLType
+    else if (arity >= 0) FunctionType(arity)
+    else if (phantomClassName(cls).isFunction) FunctionType(0)
     else NoType
   }
 
@@ -893,7 +920,10 @@ class Definitions {
    *  trait gets screwed up. Therefore, it is mandatory that FunctionXXL
    *  is treated as a NoInit trait.
    */
-  lazy val NoInitClasses = NotRuntimeClasses + FunctionXXLClass
+  private lazy val NoInitClasses = NotRuntimeClasses + FunctionXXLClass
+
+  def isNoInitClass(cls: Symbol): Boolean =
+    cls.is(NoInitsTrait) || NoInitClasses.contains(cls) || isFunctionClass(cls)
 
   def isPolymorphicAfterErasure(sym: Symbol) =
      (sym eq Any_isInstanceOf) || (sym eq Any_asInstanceOf)
@@ -914,7 +944,11 @@ class Definitions {
   def isFunctionType(tp: Type)(implicit ctx: Context) = {
     val arity = functionArity(tp)
     val sym = tp.dealias.typeSymbol
-    arity >= 0 && isFunctionClass(sym) && tp.isRef(FunctionType(arity, sym.name.isImplicitFunction).typeSymbol)
+    def top =
+      if (!sym.owner.derivesFrom(defn.PhantomClass)) defn.AnyType
+      else sym.owner.thisType.select(tpnme.Any)
+    def funType = FunctionType(arity, sym.name.isImplicitFunction, top)
+    arity >= 0 && isFunctionClass(sym) && tp.isRef(funType.typeSymbol)
   }
 
   def functionArity(tp: Type)(implicit ctx: Context) = tp.dealias.argInfos.length - 1
@@ -1030,6 +1064,10 @@ class Definitions {
 
   lazy val PhantomClass: ClassSymbol = {
     val cls = completeClass(enterCompleteClassSymbol(ScalaPackageClass, tpnme.Phantom, NoInitsTrait, List(AnyType)))
+    cls.unforcedDecls.openForMutations.useSynthesizer { name => ctx =>
+      if (name.isTypeName && name.isFunction) newFunctionNTrait(name.asTypeName, cls)
+      else NoSymbol
+    }
 
     val any = enterCompleteClassSymbol(cls, tpnme.Any, Protected | Final | NoInitsTrait, Nil)
     val nothing = enterCompleteClassSymbol(cls, tpnme.Nothing, Protected | Final | NoInitsTrait, List(any.typeRef))
@@ -1050,4 +1088,16 @@ class Definitions {
 
   def ErasedPhantom_UNIT(implicit ctx: Context) = ErasedPhantomClass.linkedClass.requiredValue("UNIT")
 
+  /** Ensure that `tp2`' is in the same universe as `tp1`. If that's the case, return
+   *  `op` applied to both types.
+   *  If not, issue an error and return `tp1`'.
+   */
+  def inSameUniverse(op: (Type, Type) => Type, tp1: Type, tp2: Type, relationship: => String, pos: Position)(implicit ctx: Context): Type =
+    if (tp1.topType == tp2.topType)
+      op(tp1, tp2)
+    else {
+      ctx.error(ex"$tp1 and $tp2 are in different universes. They cannot be combined in $relationship", pos)
+      tp1
+    }
+
 }

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

@@ -159,7 +159,7 @@ object NameOps {
       }
     }
 
-    /** Is a synthetic function name
+    /** Is a function name
      *    - N for FunctionN
      *    - N for ImplicitFunctionN (N >= 1)
       *   - (-1) otherwise
@@ -183,12 +183,12 @@ object NameOps {
      */
     def isImplicitFunction: Boolean = functionArityFor(str.ImplicitFunction) >= 1
 
-    /** Is a synthetic function name
+    /** Is a synthetic function name (in scala package)
      *    - FunctionN for N > 22
      *    - ImplicitFunctionN for N >= 1
      *    - false otherwise
      */
-    def isSyntheticFunction: Boolean = {
+    def isSyntheticScalaFunction: Boolean = {
       functionArityFor(str.Function) > MaxImplementedFunctionArity ||
         functionArityFor(str.ImplicitFunction) >= 1
     }

compiler/src/dotty/tools/dotc/transform/Mixin.scala

@@ -178,7 +178,7 @@ class Mixin extends MiniPhaseTransform with SymTransformer { thisTransform =>
       case Some(call) =>
         if (defn.NotRuntimeClasses.contains(baseCls)) Nil else call :: Nil
       case None =>
-        if (baseCls.is(NoInitsTrait) || defn.NoInitClasses.contains(baseCls)) Nil
+        if (defn.isNoInitClass(baseCls)) Nil
         else {
           //println(i"synth super call ${baseCls.primaryConstructor}: ${baseCls.primaryConstructor.info}")
           transformFollowingDeep(superRef(baseCls.primaryConstructor).appliedToNone) :: Nil

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

@@ -453,10 +453,10 @@ trait TypeAssigner {
     tree.withType(ref.tpe)
 
   def assignType(tree: untpd.AndTypeTree, left: Tree, right: Tree)(implicit ctx: Context) =
-    tree.withType(inSameUniverse(_ & _, left.tpe, right, "an `&`"))
+    tree.withType(defn.inSameUniverse(_ & _, left.tpe, right.tpe, "an `&`", right.pos))
 
   def assignType(tree: untpd.OrTypeTree, left: Tree, right: Tree)(implicit ctx: Context) =
-    tree.withType(inSameUniverse(_ | _, left.tpe, right, "an `|`"))
+    tree.withType(defn.inSameUniverse(_ | _, left.tpe, right.tpe, "an `|`", right.pos))
 
   /** Assign type of RefinedType.
    *  Refinements are typed as if they were members of refinement class `refineCls`.
@@ -491,7 +491,7 @@ trait TypeAssigner {
   def assignType(tree: untpd.TypeBoundsTree, lo: Tree, hi: Tree)(implicit ctx: Context) =
     tree.withType(
         if (lo eq hi) TypeAlias(lo.tpe)
-        else inSameUniverse(TypeBounds(_, _), lo.tpe, hi, "type bounds"))
+        else defn.inSameUniverse(TypeBounds(_, _), lo.tpe, hi.tpe, "type bounds", hi.pos))
 
   def assignType(tree: untpd.Bind, sym: Symbol)(implicit ctx: Context) =
     tree.withType(NamedType.withFixedSym(NoPrefix, sym))
@@ -537,21 +537,9 @@ trait TypeAssigner {
   def assignType(tree: untpd.PackageDef, pid: Tree)(implicit ctx: Context) =
     tree.withType(pid.symbol.valRef)
 
-  /** Ensure that `tree2`'s type is in the same universe as `tree1`. If that's the case, return
-   *  `op` applied to both types.
-   *  If not, issue an error and return `tree1`'s type.
-   */
-  private def inSameUniverse(op: (Type, Type) => Type, tp1: Type, tree2: Tree, relationship: => String)(implicit ctx: Context): Type =
-    if (tp1.topType == tree2.tpe.topType)
-      op(tp1, tree2.tpe)
-    else {
-      ctx.error(ex"$tp1 and ${tree2.tpe} are in different universes. They cannot be combined in $relationship", tree2.pos)
-      tp1
-    }
-
   private def lubInSameUniverse(trees: List[Tree], relationship: => String)(implicit ctx: Context): Type =
     trees match {
-      case first :: rest => (first.tpe /: rest)(inSameUniverse(_ | _, _, _, relationship))
+      case first :: rest => (first.tpe /: rest)((tp, tree) => defn.inSameUniverse(_ | _, tp, tree.tpe, relationship, tree.pos))
       case Nil => defn.NothingType
     }
 }

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

@@ -704,9 +704,9 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
           else true
         case _ => false
       }
-      val funCls = defn.FunctionClass(args.length, isImplicit)
-      typed(cpy.AppliedTypeTree(tree)(
-        untpd.TypeTree(funCls.typeRef), args :+ body), pt)
+      val argsTypes = args.map(tr => typed(tr).tpe)
+      val funTpe = defn.FunctionType(argsTypes, typed(body).tpe, isImplicit)
+      typed(cpy.AppliedTypeTree(tree)(untpd.TypeTree(funTpe), args :+ body), pt)
     }
     else {
       val params = args.asInstanceOf[List[untpd.ValDef]]

library/src/scala/Phantom.scala

@@ -8,5 +8,20 @@ trait Phantom {
   protected final trait Nothing extends this.Any
 
   protected final def assume: this.Nothing
+
+  trait Function1[-T1 <: this.Any, +R] {
+    def apply(x1: T1): R
+  }
+  trait ImplicitFunction1[-T1 <: this.Any, +R] extends Function1[T1, R] {
+    /*implicit*/ def apply(x1: T1): R
+  }
+  ...
+  trait FunctionN[-T1 <: this.Any, ..., -Tn <: this.Any, +R] {
+    def apply(x1: T1, ..., xn: Tn): R
+  }
+  trait ImplicitFunctionN[-T1 <: this.Any, ..., -Tn <: this.Any, +R] extends FunctionN[T1, ..., Tn, R] {
+    /*implicit*/ def apply(x1: T1, ..., xn: Tn): R
+  }
+
 }
 */

tests/neg/phantom-Functions-1.scala

@@ -0,0 +1,6 @@
+
+class PhantomFun1NoApply extends Function1[Boo.Casper, Unit] // error: class PhantomFun1NoApply needs to be abstract, since def apply: (p0: Casper)Unit is not defined
+
+object Boo extends Phantom {
+  type Casper <: this.Any
+}

tests/neg/phantom-Functions-2.scala

@@ -0,0 +1,16 @@
+
+class PhantomFun1 extends Boo.Function2[Boo.Casper, Int, Unit] { // error: Type argument Int does not conform to upper bound Boo.Any
+  def apply(x1: Boo.Casper, x2: Int): Unit = ()
+}
+
+class PhantomFun2 extends Boo.Function2[Int, Boo.Casper, Unit] { // error: Type argument Int does not conform to upper bound Boo.Any
+  def apply(x1: Boo.Casper, x2: Int): Unit = ()
+}
+
+class Fun extends Function2[Int, Int, Unit] {
+  def apply(x1: Int, x2: Int): Unit = ()
+}
+
+object Boo extends Phantom {
+  type Casper <: this.Any
+}

tests/neg/phantom-Functions-3.scala

@@ -0,0 +1,14 @@
+
+// TODO: Importing an object that exends phantom makes FunctionN refere to the Boo.FunctionN
+//       We should be carefull with this. Use a waring when importing Boo._ or disallow it.
+//       Or put funtions in an inner object in the Phantom trait, for example scala.Phantom.Functions
+
+import Boo._
+
+class Fun extends Function1[Int, Unit] { // error: Type argument Int does not conform to upper bound Boo.Any
+  def apply(x1: Int): Unit = ()
+}
+
+object Boo extends Phantom {
+  type Casper <: this.Any
+}

tests/neg/phantom-Lambda.scala

@@ -0,0 +1,16 @@
+
+class Fun {
+  def a = (x1: Int, x2: Boo.B) => x1 // error: Int and Boo.B are in different universes. They cannot be combined in function arguments.
+  def b: (Int, Boo.B) => Unit = (x1, x2) => x1 // error: Int and Boo.B are in different universes. They cannot be combined in function arguments.
+
+  def c = (x1: Foo.F, x2: Boo.B) => x1 // error: Foo.F and Boo.B are in different universes. They cannot be combined in function arguments.
+  def d: (Foo.F, Boo.B) => Unit = (x1, x2) => x1 // error: Foo.F and Boo.B are in different universes. They cannot be combined in function arguments.
+}
+
+object Boo extends Phantom {
+  type B <: this.Any
+}
+
+object Foo extends Phantom {
+  type F <: this.Any
+}