Implement phantom types part 1

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

@@ -670,16 +670,6 @@ object desugar {
       tree
   }
 
-  /** EmptyTree in lower bound ==> Nothing
-   *  EmptyTree in upper bounds ==> Any
-   */
-  def typeBoundsTree(tree: TypeBoundsTree)(implicit ctx: Context): TypeBoundsTree = {
-    val TypeBoundsTree(lo, hi) = tree
-    val lo1 = if (lo.isEmpty) untpd.TypeTree(defn.NothingType) else lo
-    val hi1 = if (hi.isEmpty) untpd.TypeTree(defn.AnyType) else hi
-    cpy.TypeBoundsTree(tree)(lo1, hi1)
-  }
-
   /** Make closure corresponding to function.
    *      params => body
    *  ==>

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

@@ -820,7 +820,7 @@ class Definitions {
   lazy val UnqualifiedOwnerTypes: Set[NamedType] =
     RootImportTypes.toSet[NamedType] ++ RootImportTypes.map(_.symbol.moduleClass.typeRef)
 
-  lazy val PhantomClasses = Set[Symbol](AnyClass, AnyValClass, NullClass, NothingClass)
+  lazy val NotRuntimeClasses = Set[Symbol](AnyClass, AnyValClass, NullClass, NothingClass)
 
   /** Classes that are known not to have an initializer irrespective of
    *  whether NoInits is set. Note: FunctionXXLClass is in this set
@@ -832,7 +832,7 @@ class Definitions {
    *  trait gets screwed up. Therefore, it is mandatory that FunctionXXL
    *  is treated as a NoInit trait.
    */
-  lazy val NoInitClasses = PhantomClasses + FunctionXXLClass
+  lazy val NoInitClasses = NotRuntimeClasses + FunctionXXLClass
 
   def isPolymorphicAfterErasure(sym: Symbol) =
      (sym eq Any_isInstanceOf) || (sym eq Any_asInstanceOf)
@@ -936,7 +936,8 @@ class Definitions {
     NullClass,
     NothingClass,
     SingletonClass,
-    EqualsPatternClass)
+    EqualsPatternClass,
+    PhantomClass)
 
   lazy val syntheticCoreClasses = syntheticScalaClasses ++ List(
     EmptyPackageVal,
@@ -963,4 +964,23 @@ class Definitions {
       _isInitialized = true
     }
   }
+
+  // ----- Phantoms ---------------------------------------------------------
+
+  lazy val PhantomClass: ClassSymbol = {
+    val cls = completeClass(enterCompleteClassSymbol(ScalaPackageClass, tpnme.Phantom, NoInitsTrait, List(AnyType)))
+
+    val any = enterCompleteClassSymbol(cls, tpnme.Any, Protected | Final | NoInitsTrait, Nil)
+    val nothing = enterCompleteClassSymbol(cls, tpnme.Nothing, Protected | Final | NoInitsTrait, List(any.typeRef))
+    enterMethod(cls, nme.assume_, MethodType(Nil, nothing.typeRef), Protected | Final | Method)
+
+    cls
+  }
+  lazy val Phantom_AnyClass = PhantomClass.unforcedDecls.find(_.name eq tpnme.Any).asClass
+  lazy val Phantom_NothingClass = PhantomClass.unforcedDecls.find(_.name eq tpnme.Nothing).asClass
+  lazy val Phantom_assume = PhantomClass.unforcedDecls.find(_.name eq nme.assume_)
+
+  /** If the symbol is of the class scala.Phantom.Any or scala.Phantom.Nothing */
+  def isPhantomTerminalClass(sym: Symbol) = (sym eq Phantom_AnyClass) || (sym eq Phantom_NothingClass)
+
 }

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

@@ -0,0 +1,28 @@
+package dotty.tools.dotc.core
+
+import dotty.tools.dotc.ast.tpd._
+import dotty.tools.dotc.core.Contexts.Context
+import dotty.tools.dotc.core.Symbols.defn
+import dotty.tools.dotc.core.Types.Type
+
+/** Phantom erasure erases (minimal erasure):
+ *
+ *  - Parameters/arguments are erased to BoxedUnit. The next step will remove the parameters
+ *    from the method definitions and calls (implemented in branch implement-phantom-types-part-2).
+ *  - Definitions of `def`, `val`, `lazy val` and `var` returning a phantom type to return a BoxedUnit. The next step
+ *    is to erase the fields for phantom types (implemented in branch implement-phantom-types-part-3)
+ *  - Calls to Phantom.assume become calls to BoxedUnit. Intended to be optimized away by local optimizations.
+ *
+ *  BoxedUnit is used as it fits perfectly and homogeneously in all locations where phantoms can be found.
+ *  Additionally some of the optimizations that can be performed on phantom types can also be directly implemented
+ *  on all boxed units.
+ */
+object PhantomErasure {
+
+  /** Returns the default erased type of a phantom type */
+  def erasedPhantomType(implicit ctx: Context): Type = defn.BoxedUnitType
+
+  /** Returns the default erased tree for a call to Phantom.assume */
+  def erasedAssume(implicit ctx: Context): Tree = ref(defn.BoxedUnit_UNIT)
+
+}

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

@@ -237,6 +237,8 @@ object StdNames {
     final val SourceFileATTR: N             = "SourceFile"
     final val SyntheticATTR: N              = "Synthetic"
 
+    final val Phantom: N = "Phantom"
+
 // ----- Term names -----------------------------------------
 
     // Compiler-internal

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

@@ -483,8 +483,8 @@ object SymDenotations {
     def isNumericValueClass(implicit ctx: Context) =
       maybeOwner == defn.ScalaPackageClass && defn.ScalaNumericValueClasses().contains(symbol)
 
-    /** Is symbol a phantom class for which no runtime representation exists? */
-    def isPhantomClass(implicit ctx: Context) = defn.PhantomClasses contains symbol
+    /** Is symbol a class for which no runtime representation exists? */
+    def isNotRuntimeClass(implicit ctx: Context) = defn.NotRuntimeClasses contains symbol
 
     /** Is this symbol a class representing a refinement? These classes
      *  are used only temporarily in Typer and Unpickler as an intermediate
@@ -635,7 +635,7 @@ object SymDenotations {
 
     /** Is this symbol a class references to which that are supertypes of null? */
     final def isNullableClass(implicit ctx: Context): Boolean =
-      isClass && !isValueClass && !(this is ModuleClass) && symbol != defn.NothingClass
+      isClass && !isValueClass && !(this is ModuleClass) && symbol != defn.NothingClass && !defn.isPhantomTerminalClass(symbol)
 
     /** Is this definition accessible as a member of tree with type `pre`?
      *  @param pre          The type of the tree from which the selection is made

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

@@ -47,6 +47,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
   private var myAnyClass: ClassSymbol = null
   private var myNothingClass: ClassSymbol = null
   private var myNullClass: ClassSymbol = null
+  private var myPhantomNothingClass: ClassSymbol = null
   private var myObjectClass: ClassSymbol = null
   private var myAnyType: TypeRef = null
   private var myNothingType: TypeRef = null
@@ -63,6 +64,10 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
     if (myNullClass == null) myNullClass = defn.NullClass
     myNullClass
   }
+  def PhantomNothingClass = {
+    if (myPhantomNothingClass == null) myPhantomNothingClass = defn.Phantom_NothingClass
+    myPhantomNothingClass
+  }
   def ObjectClass = {
     if (myObjectClass == null) myObjectClass = defn.ObjectClass
     myObjectClass
@@ -550,8 +555,10 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
             case OrType(tp1, tp2) => isNullable(tp1) || isNullable(tp2)
             case _ => false
           }
-          (tp1.symbol eq NothingClass) && tp2.isValueTypeOrLambda ||
-          (tp1.symbol eq NullClass) && isNullable(tp2)
+          val sym1 = tp1.symbol
+          (sym1 eq NothingClass) && tp2.isValueTypeOrLambda && !tp2.isPhantom ||
+          (sym1 eq NullClass) && isNullable(tp2) ||
+          (sym1 eq PhantomNothingClass) && tp1.topType == tp2.topType
       }
     case tp1: SingletonType =>
       /** if `tp2 == p.type` and `p: q.type` then try `tp1 <:< q.type` as a last effort.*/

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

@@ -364,6 +364,7 @@ class TypeErasure(isJava: Boolean, semiEraseVCs: Boolean, isConstructor: Boolean
       else if (semiEraseVCs && isDerivedValueClass(sym)) eraseDerivedValueClassRef(tp)
       else if (sym == defn.ArrayClass) apply(tp.appliedTo(TypeBounds.empty)) // i966 shows that we can hit a raw Array type.
       else if (defn.isSyntheticFunctionClass(sym)) defn.erasedFunctionType(sym)
+      else if (defn.isPhantomTerminalClass(tp.symbol)) PhantomErasure.erasedPhantomType
       else eraseNormalClassRef(tp)
     case tp: RefinedType =>
       val parent = tp.parent
@@ -403,7 +404,8 @@ class TypeErasure(isJava: Boolean, semiEraseVCs: Boolean, isConstructor: Boolean
             case tr :: trs1 =>
               assert(!tr.classSymbol.is(Trait), cls)
               val tr1 = if (cls is Trait) defn.ObjectType else tr
-              tr1 :: trs1.filterNot(_ isRef defn.ObjectClass)
+              // We remove the Phantom trait to erase the definitions of Phantom.{assume, Any, Nothing}
+              tr1 :: trs1.filterNot(x => x.isRef(defn.ObjectClass) || x.isRef(defn.PhantomClass))
             case nil => nil
           }
         val erasedDecls = decls.filteredScope(sym => !sym.isType || sym.isClass)
@@ -506,6 +508,8 @@ class TypeErasure(isJava: Boolean, semiEraseVCs: Boolean, isConstructor: Boolean
         }
         if (defn.isSyntheticFunctionClass(sym))
           sigName(defn.erasedFunctionType(sym))
+        else if (defn.isPhantomTerminalClass(tp.symbol))
+          sigName(PhantomErasure.erasedPhantomType)
         else
           normalizeClass(sym.asClass).fullName.asTypeName
       case defn.ArrayOf(elem) =>

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

@@ -172,7 +172,44 @@ object Types {
         case _ =>
           false
       }
-      cls == defn.AnyClass || loop(this)
+      loop(this)
+    }
+
+    /** Returns true if the type is a phantom type
+     *   - true if XYZ extends scala.Phantom and this type is upper bounded XYZ.Any
+     *   - false otherwise
+     */
+    final def isPhantom(implicit ctx: Context): Boolean = phantomLatticeType.exists
+
+    /** Returns the top type of the lattice
+     *   - XYX.Any if XYZ extends scala.Phantom and this type is upper bounded XYZ.Any
+     *   - scala.Any otherwise
+     */
+    final def topType(implicit ctx: Context): Type = {
+      val lattice = phantomLatticeType
+      if (lattice.exists) lattice.select(tpnme.Any)
+      else defn.AnyType
+    }
+
+    /** Returns the bottom type of the lattice
+     *   - XYZ.Nothing if XYZ extends scala.Phantom and this type is upper bounded XYZ.Any
+     *   - scala.Nothing otherwise
+     */
+    final def bottomType(implicit ctx: Context): Type = {
+      val lattice = phantomLatticeType
+      if (lattice.exists) lattice.select(tpnme.Nothing)
+      else defn.NothingType
+    }
+
+    /** Returns the type of the phantom lattice (i.e. the prefix of the phantom type)
+      *   - XYZ if XYZ extends scala.Phantom and this type is upper bounded XYZ.Any
+      *   - NoType otherwise
+      */
+    private final def phantomLatticeType(implicit ctx: Context): Type = widen match {
+      case tp: ClassInfo if defn.isPhantomTerminalClass(tp.classSymbol) => tp.prefix
+      case tp: TypeProxy if tp.superType ne this => tp.underlying.phantomLatticeType
+      case tp: AndOrType => tp.tp1.phantomLatticeType
+      case _ => NoType
     }
 
     /** Is this type guaranteed not to have `null` as a value?

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

@@ -89,7 +89,7 @@ class ElimRepeated extends MiniPhaseTransform with InfoTransformer with Annotati
     case _ =>
       val elemType = tree.tpe.elemType
       var elemClass = elemType.classSymbol
-      if (defn.PhantomClasses contains elemClass) elemClass = defn.ObjectClass
+      if (defn.NotRuntimeClasses contains elemClass) elemClass = defn.ObjectClass
       ref(defn.DottyArraysModule)
         .select(nme.seqToArray)
         .appliedToType(elemType)

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

@@ -28,6 +28,7 @@ import ValueClasses._
 import TypeUtils._
 import ExplicitOuter._
 import core.Mode
+import core.PhantomErasure
 
 class Erasure extends Phase with DenotTransformer { thisTransformer =>
 
@@ -454,6 +455,8 @@ object Erasure extends TypeTestsCasts{
       val Apply(fun, args) = tree
       if (fun.symbol == defn.cbnArg)
         typedUnadapted(args.head, pt)
+      else if (fun.symbol eq defn.Phantom_assume)
+        PhantomErasure.erasedAssume
       else typedExpr(fun, FunProto(args, pt, this)) match {
         case fun1: Apply => // arguments passed in prototype were already passed
           fun1

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

@@ -174,7 +174,7 @@ class Mixin extends MiniPhaseTransform with SymTransformer { thisTransform =>
 
     def superCallOpt(baseCls: Symbol): List[Tree] = superCalls.get(baseCls) match {
       case Some(call) =>
-        if (defn.PhantomClasses.contains(baseCls)) Nil else call :: Nil
+        if (defn.NotRuntimeClasses.contains(baseCls)) Nil else call :: Nil
       case None =>
         if (baseCls.is(NoInitsTrait) || defn.NoInitClasses.contains(baseCls)) Nil
         else {

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

@@ -14,7 +14,7 @@ object TreeGen {
   def wrapArrayMethodName(elemtp: Type)(implicit ctx: Context): TermName = {
     val elemCls = elemtp.classSymbol
     if (elemCls.isPrimitiveValueClass) nme.wrapXArray(elemCls.name)
-    else if (elemCls.derivesFrom(defn.ObjectClass) && !elemCls.isPhantomClass) nme.wrapRefArray
+    else if (elemCls.derivesFrom(defn.ObjectClass) && !elemCls.isNotRuntimeClass) nme.wrapRefArray
     else nme.genericWrapArray
   }
 

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

@@ -418,7 +418,7 @@ trait TypeAssigner {
     tree.withType(avoidingType(expansion, bindings))
 
   def assignType(tree: untpd.If, thenp: Tree, elsep: Tree)(implicit ctx: Context) =
-    tree.withType(thenp.tpe | elsep.tpe)
+    tree.withType(lubInSameUniverse(thenp :: elsep :: Nil, "branches of an if/else"))
 
   def assignType(tree: untpd.Closure, meth: Tree, target: Tree)(implicit ctx: Context) =
     tree.withType(
@@ -428,15 +428,18 @@ trait TypeAssigner {
   def assignType(tree: untpd.CaseDef, body: Tree)(implicit ctx: Context) =
     tree.withType(body.tpe)
 
-  def assignType(tree: untpd.Match, cases: List[CaseDef])(implicit ctx: Context) =
-    tree.withType(ctx.typeComparer.lub(cases.tpes))
+  def assignType(tree: untpd.Match, cases: List[CaseDef])(implicit ctx: Context) = {
+    if (tree.selector.typeOpt.isPhantom)
+      ctx.error("cannot pattern match on values of a phantom type", tree.selector.pos)
+    tree.withType(lubInSameUniverse(cases, "branches of a match"))
+  }
 
   def assignType(tree: untpd.Return)(implicit ctx: Context) =
     tree.withType(defn.NothingType)
 
   def assignType(tree: untpd.Try, expr: Tree, cases: List[CaseDef])(implicit ctx: Context) =
     if (cases.isEmpty) tree.withType(expr.tpe)
-    else tree.withType(ctx.typeComparer.lub(expr.tpe :: cases.tpes))
+    else tree.withType(lubInSameUniverse(expr :: cases, "branches of a try"))
 
   def assignType(tree: untpd.SeqLiteral, elems: List[Tree], elemtpt: Tree)(implicit ctx: Context) = {
     val ownType = tree match {
@@ -450,10 +453,10 @@ trait TypeAssigner {
     tree.withType(ref.tpe)
 
   def assignType(tree: untpd.AndTypeTree, left: Tree, right: Tree)(implicit ctx: Context) =
-    tree.withType(left.tpe & right.tpe)
+    tree.withType(inSameUniverse(_ & _, left.tpe, right, "an `&`"))
 
   def assignType(tree: untpd.OrTypeTree, left: Tree, right: Tree)(implicit ctx: Context) =
-    tree.withType(left.tpe | right.tpe)
+    tree.withType(inSameUniverse(_ | _, left.tpe, right, "an `|`"))
 
   /** Assign type of RefinedType.
    *  Refinements are typed as if they were members of refinement class `refineCls`.
@@ -484,7 +487,9 @@ trait TypeAssigner {
     tree.withType(ExprType(result.tpe))
 
   def assignType(tree: untpd.TypeBoundsTree, lo: Tree, hi: Tree)(implicit ctx: Context) =
-    tree.withType(if (lo eq hi) TypeAlias(lo.tpe) else TypeBounds(lo.tpe, hi.tpe))
+    tree.withType(
+        if (lo eq hi) TypeAlias(lo.tpe)
+        else inSameUniverse(TypeBounds(_, _), lo.tpe, hi, "type bounds"))
 
   def assignType(tree: untpd.Bind, sym: Symbol)(implicit ctx: Context) =
     tree.withType(NamedType.withFixedSym(NoPrefix, sym))
@@ -529,6 +534,24 @@ 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 Nil => defn.NothingType
+    }
 }
 
 object TypeAssigner extends TypeAssigner

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

@@ -1108,10 +1108,14 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
   }
 
   def typedTypeBoundsTree(tree: untpd.TypeBoundsTree)(implicit ctx: Context): TypeBoundsTree = track("typedTypeBoundsTree") {
-    val TypeBoundsTree(lo, hi) = desugar.typeBoundsTree(tree)
+    val TypeBoundsTree(lo, hi) = tree
     val lo1 = typed(lo)
     val hi1 = typed(hi)
-    val tree1 = assignType(cpy.TypeBoundsTree(tree)(lo1, hi1), lo1, hi1)
+
+    val lo2 = if (lo1.isEmpty) typed(untpd.TypeTree(hi1.typeOpt.bottomType)) else lo1
+    val hi2 = if (hi1.isEmpty) typed(untpd.TypeTree(lo1.typeOpt.topType)) else hi1
+
+    val tree1 = assignType(cpy.TypeBoundsTree(tree)(lo2, hi2), lo2, hi2)
     if (ctx.mode.is(Mode.Pattern)) {
       // Associate a pattern-bound type symbol with the wildcard.
       // The bounds of the type symbol can be constrained when comparing a pattern type
@@ -1330,6 +1334,10 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
           cls, isRequired, cdef.pos)
     }
 
+    // Check that phantom lattices are defined in a static object
+    if (cls.classParents.exists(_.classSymbol eq defn.PhantomClass) && !cls.isStaticOwner)
+      ctx.error("only static objects can extend scala.Phantom", cdef.pos)
+
     // check value class constraints
     checkDerivedValueClass(cls, body1)
 

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -152,6 +152,7 @@ class CompilationTests extends ParallelTesting {
     compileFile("../tests/neg/customArgs/noimports2.scala", defaultOptions.and("-Yno-imports")) +
     compileFile("../tests/neg/customArgs/overloadsOnAbstractTypes.scala", allowDoubleBindings) +
     compileFile("../tests/neg/customArgs/xfatalWarnings.scala", defaultOptions.and("-Xfatal-warnings")) +
+    compileFile("../tests/neg/customArgs/phantom-overload.scala", allowDoubleBindings) +
     compileFile("../tests/neg/tailcall/t1672b.scala", defaultOptions) +
     compileFile("../tests/neg/tailcall/t3275.scala", defaultOptions) +
     compileFile("../tests/neg/tailcall/t6574.scala", defaultOptions) +