Use PolyFunction instead of ErasedFunction

We generalize the meaning of `PolyFunction` to mean any kind of refined
lambda encoding. These refinements support any type with the following
shape as a lambda type:

```scala
PolyFunction {
  def apply[[T1, ..., Tn]]([given] [erased] x1: X1, ..., [erased] xn: Xn): R
}
```

Author: nicolasstucki

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

@@ -954,7 +954,7 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
   def isStructuralTermSelectOrApply(tree: Tree)(using Context): Boolean = {
     def isStructuralTermSelect(tree: Select) =
       def hasRefinement(qualtpe: Type): Boolean = qualtpe.dealias match
-        case defn.PolyOrErasedFunctionOf(_) =>
+        case defn.PolyFunctionOf(_) =>
           false
         case RefinedType(parent, rname, rinfo) =>
           rname == tree.name || hasRefinement(parent)

compiler/src/dotty/tools/dotc/cc/Setup.scala

@@ -197,7 +197,7 @@ extends tpd.TreeTraverser:
       val mt = ContextualMethodType(paramName :: Nil)(
         _ => paramType :: Nil,
         mt => if isLast then res else expandThrowsAlias(res, mt :: encl))
-      val fntpe = RefinedType(defn.ErasedFunctionClass.typeRef, nme.apply, mt)
+      val fntpe = RefinedType(defn.PolyFunctionClass.typeRef, nme.apply, mt)
       if !encl.isEmpty && isLast then
         val cs = CaptureSet(encl.map(_.paramRefs.head)*)
         CapturingType(fntpe, cs, boxed = false)

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

@@ -1112,12 +1112,12 @@ class Definitions {
     def apply(args: List[Type], resultType: Type, isContextual: Boolean = false)(using Context): Type =
       val mt = MethodType.companion(isContextual, false)(args, resultType)
       if mt.hasErasedParams then
-        RefinedType(ErasedFunctionClass.typeRef, nme.apply, mt)
+        RefinedType(PolyFunctionClass.typeRef, nme.apply, mt)
       else
         FunctionType(args.length, isContextual).appliedTo(args ::: resultType :: Nil)
     def unapply(ft: Type)(using Context): Option[(List[Type], Type, Boolean)] = {
       ft.dealias match
-        case ErasedFunctionOf(mt) =>
+        case PolyFunctionOf(mt: MethodType) =>
           Some(mt.paramInfos, mt.resType, mt.isContextualMethod)
         case dft =>
           val tsym = dft.typeSymbol
@@ -1129,38 +1129,14 @@ class Definitions {
     }
   }
 
-  object PolyOrErasedFunctionOf {
-    /** Matches a refined `PolyFunction` or `ErasedFunction` type and extracts the apply info.
-     *
-     *  Pattern: `(PolyFunction | ErasedFunction) { def apply: $mt }`
-     */
-    def unapply(ft: Type)(using Context): Option[MethodicType] = ft.dealias match
-      case RefinedType(parent, nme.apply, mt: MethodicType)
-      if parent.derivesFrom(defn.PolyFunctionClass) || parent.derivesFrom(defn.ErasedFunctionClass) =>
-        Some(mt)
-      case _ => None
-  }
-
   object PolyFunctionOf {
     /** Matches a refined `PolyFunction` type and extracts the apply info.
      *
-     *  Pattern: `PolyFunction { def apply: $pt }`
+     *  Pattern: `PolyFunction { def apply: $mt }`
      */
-    def unapply(ft: Type)(using Context): Option[PolyType] = ft.dealias match
-      case RefinedType(parent, nme.apply, pt: PolyType)
+    def unapply(ft: Type)(using Context): Option[MethodicType] = ft.dealias match
+      case RefinedType(parent, nme.apply, mt: MethodicType)
       if parent.derivesFrom(defn.PolyFunctionClass) =>
-        Some(pt)
-      case _ => None
-  }
-
-  object ErasedFunctionOf {
-    /** Matches a refined `ErasedFunction` type and extracts the apply info.
-     *
-     *  Pattern: `ErasedFunction { def apply: $mt }`
-     */
-    def unapply(ft: Type)(using Context): Option[MethodType] = ft.dealias match
-      case RefinedType(parent, nme.apply, mt: MethodType)
-      if parent.derivesFrom(defn.ErasedFunctionClass) =>
         Some(mt)
       case _ => None
   }
@@ -1514,9 +1490,6 @@ class Definitions {
   lazy val PolyFunctionClass = requiredClass("scala.PolyFunction")
   def PolyFunctionType = PolyFunctionClass.typeRef
 
-  lazy val ErasedFunctionClass = requiredClass("scala.runtime.ErasedFunction")
-  def ErasedFunctionType = ErasedFunctionClass.typeRef
-
   /** If `cls` is a class in the scala package, its name, otherwise EmptyTypeName */
   def scalaClassName(cls: Symbol)(using Context): TypeName = cls.denot match
     case clsd: ClassDenotation if clsd.owner eq ScalaPackageClass =>
@@ -1579,8 +1552,6 @@ class Definitions {
   /** Is a synthetic function class
    *    - FunctionN for N > 22
    *    - ContextFunctionN for N >= 0
-   *    - ErasedFunctionN for N > 0
-   *    - ErasedContextFunctionN for N > 0
    */
   def isSyntheticFunctionClass(cls: Symbol): Boolean = scalaClassName(cls).isSyntheticFunction
 
@@ -1596,8 +1567,6 @@ class Definitions {
    *    - FunctionN for 22 > N >= 0 remains as FunctionN
    *    - ContextFunctionN for N > 22 becomes FunctionXXL
    *    - ContextFunctionN for N <= 22 becomes FunctionN
-   *    - ErasedFunctionN becomes Function0
-   *    - ImplicitErasedFunctionN becomes Function0
    *    - anything else becomes a NoType
    */
   def functionTypeErasure(cls: Symbol): Type =
@@ -1756,13 +1725,11 @@ class Definitions {
   /** Returns whether `tp` is an instance or a refined instance of:
    *  - scala.FunctionN
    *  - scala.ContextFunctionN
-   *  - ErasedFunction
    *  - PolyFunction
    */
   def isFunctionType(tp: Type)(using Context): Boolean =
     isFunctionNType(tp)
     || tp.derivesFrom(defn.PolyFunctionClass)   // TODO check for refinement?
-    || tp.derivesFrom(defn.ErasedFunctionClass) // TODO check for refinement?
 
   private def withSpecMethods(cls: ClassSymbol, bases: List[Name], paramTypes: Set[TypeRef]) =
     if !ctx.settings.Yscala2Stdlib.value then
@@ -1866,7 +1833,7 @@ class Definitions {
     tp.stripTypeVar.dealias match
       case tp1: TypeParamRef if ctx.typerState.constraint.contains(tp1) =>
         asContextFunctionType(TypeComparer.bounds(tp1).hiBound)
-      case tp1 @ ErasedFunctionOf(mt) if mt.isContextualMethod =>
+      case tp1 @ PolyFunctionOf(mt: MethodType) if mt.isContextualMethod =>
         tp1
       case tp1 =>
         if tp1.typeSymbol.name.isContextFunction && isFunctionNType(tp1) then tp1
@@ -1886,21 +1853,14 @@ class Definitions {
         atPhase(erasurePhase)(unapply(tp))
       else
         asContextFunctionType(tp) match
-          case ErasedFunctionOf(mt) =>
+          case PolyFunctionOf(mt: MethodType) =>
             Some((mt.paramInfos, mt.resType, mt.erasedParams))
           case tp1 if tp1.exists =>
             val args = tp1.functionArgInfos
-            val erasedParams = erasedFunctionParameters(tp1)
+            val erasedParams = List.fill(functionArity(tp1)) { false }
             Some((args.init, args.last, erasedParams))
           case _ => None
 
-  /* Returns a list of erased booleans marking whether parameters are erased, for a function type. */
-  def erasedFunctionParameters(tp: Type)(using Context): List[Boolean] = tp.dealias match {
-    case ErasedFunctionOf(mt) => mt.erasedParams
-    case tp if isFunctionNType(tp) => List.fill(functionArity(tp)) { false }
-    case _ => Nil
-  }
-
   /** A whitelist of Scala-2 classes that are known to be pure */
   def isAssuredNoInits(sym: Symbol): Boolean =
     (sym `eq` SomeClass) || isTupleClass(sym)

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

@@ -34,7 +34,6 @@ object StdNames {
     inline val MODULE_INSTANCE_FIELD      = "MODULE$"
 
     inline val Function                   = "Function"
-    inline val ErasedFunction             = "ErasedFunction"
     inline val ContextFunction            = "ContextFunction"
     inline val ErasedContextFunction      = "ErasedContextFunction"
     inline val AbstractFunction           = "AbstractFunction"
@@ -214,7 +213,6 @@ object StdNames {
     final val Throwable: N           = "Throwable"
     final val IOOBException: N       = "IndexOutOfBoundsException"
     final val FunctionXXL: N         = "FunctionXXL"
-    final val ErasedFunction: N      = "ErasedFunction"
 
     final val Abs: N                  = "Abs"
     final val And: N                  = "&&"

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

@@ -506,10 +506,10 @@ class TypeApplications(val self: Type) extends AnyVal {
     case _ => Nil
 
   /** If this is an encoding of a function type, return its arguments, otherwise return Nil.
-   *  Handles `ErasedFunction`s and poly functions gracefully.
+   *  Handles poly functions gracefully.
    */
   final def functionArgInfos(using Context): List[Type] = self.dealias match
-    case defn.ErasedFunctionOf(mt) => (mt.paramInfos :+ mt.resultType)
+    case defn.PolyFunctionOf(mt: MethodType) => (mt.paramInfos :+ mt.resultType)
     case _ => self.dropDependentRefinement.dealias.argInfos
 
   /** Argument types where existential types in arguments are disallowed */

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

@@ -667,7 +667,6 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
 
             if defn.isFunctionType(tp2) then
               if tp2.derivesFrom(defn.PolyFunctionClass) then
-                // TODO should we handle ErasedFunction is this same way?
                 tp1.member(nme.apply).info match
                   case info1: PolyType =>
                     return isSubInfo(info1, tp2.refinedInfo)

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

@@ -560,7 +560,7 @@ object TypeErasure {
     case _ => false
   }
 
-  /** The erasure of `(PolyFunction | ErasedFunction) { def apply: $applyInfo }` */
+  /** The erasure of `PolyFunction { def apply: $applyInfo }` */
   def eraseRefinedFunctionApply(applyInfo: Type)(using Context): Type =
     def functionType(info: Type): Type = info match {
       case info: PolyType =>
@@ -654,7 +654,7 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst
         else SuperType(eThis, eSuper)
       case ExprType(rt) =>
         defn.FunctionType(0)
-      case defn.PolyOrErasedFunctionOf(mt) =>
+      case defn.PolyFunctionOf(mt) =>
         eraseRefinedFunctionApply(mt)
       case tp: TypeVar if !tp.isInstantiated =>
         assert(inSigName, i"Cannot erase uninstantiated type variable $tp")
@@ -936,7 +936,7 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst
         sigName(defn.FunctionOf(Nil, rt))
       case tp: TypeVar if !tp.isInstantiated =>
         tpnme.Uninstantiated
-      case tp @ defn.PolyOrErasedFunctionOf(_) =>
+      case tp @ defn.PolyFunctionOf(_) =>
         // we need this case rather than falling through to the default
         // because RefinedTypes <: TypeProxy and it would be caught by
         // the case immediately below

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

@@ -1886,7 +1886,7 @@ object Types {
             formals1 mapConserve (_.translateFromRepeated(toArray = isJava)),
             result1, isContextual)
         if mt.hasErasedParams then
-          RefinedType(defn.ErasedFunctionType, nme.apply, mt)
+          RefinedType(defn.PolyFunctionType, nme.apply, mt)
         else if alwaysDependent || mt.isResultDependent then
           RefinedType(nonDependentFunType, nme.apply, mt)
         else nonDependentFunType

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

@@ -679,7 +679,7 @@ object Erasure {
           // Instead, we manually lookup the type of `apply` in the qualifier.
           inContext(preErasureCtx) {
             val qualTp = tree.qualifier.typeOpt.widen
-            if qualTp.derivesFrom(defn.PolyFunctionClass) || qualTp.derivesFrom(defn.ErasedFunctionClass) then
+            if qualTp.derivesFrom(defn.PolyFunctionClass) then
               eraseRefinedFunctionApply(qualTp.select(nme.apply).widen).classSymbol
             else
               NoSymbol

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

@@ -446,11 +446,8 @@ object TreeChecker {
       assert(tree.isTerm || !ctx.isAfterTyper, tree.show + " at " + ctx.phase)
       val tpe = tree.typeOpt
 
-      // PolyFunction and ErasedFunction apply methods stay structural until Erasure
-      val isRefinedFunctionApply = (tree.name eq nme.apply) && {
-        val qualTpe = tree.qualifier.typeOpt
-        qualTpe.derivesFrom(defn.PolyFunctionClass) || qualTpe.derivesFrom(defn.ErasedFunctionClass)
-      }
+      // PolyFunction apply method stay structural until Erasure
+      val isRefinedFunctionApply = (tree.name eq nme.apply) && tree.qualifier.typeOpt.derivesFrom(defn.PolyFunctionClass)
 
       // Outer selects are pickled specially so don't require a symbol
       val isOuterSelect = tree.name.is(OuterSelectName)