Merge pull request #1758 from dotty-staging/change-functions

Drop function 22 limit

1 files changed, 137 insertions, 77 deletions

diff --git a/compiler/src/dotty/tools/dotc/core/Definitions.scala b/compiler/src/dotty/tools/dotc/core/Definitions.scala
index c5ccab261..e4e5761b2 100644
--- a/compiler/src/dotty/tools/dotc/core/Definitions.scala
+++ b/compiler/src/dotty/tools/dotc/core/Definitions.scala
@@ -12,13 +12,20 @@ import collection.mutable
 import scala.reflect.api.{ Universe => ApiUniverse }
 
 object Definitions {
-  val MaxTupleArity, MaxAbstractFunctionArity = 22
-  val MaxFunctionArity = 30
-    // Awaiting a definite solution that drops the limit altogether, 30 gives a safety
-    // margin over the previous 22, so that treecopiers in miniphases are allowed to
-    // temporarily create larger closures. This is needed in lambda lift where large closures
-    // are first formed by treecopiers before they are split apart into parameters and
-    // environment in the lambdalift transform itself.
+
+  /** The maximum number of elements in a tuple or product.
+   *  This should be removed once we go to hlists.
+   */
+  val MaxTupleArity = 22
+
+  /** The maximum arity N of a function type that's implemented
+   *  as a trait `scala.FunctionN`. Functions of higher arity are possible,
+   *  but are mapped in erasure to functions taking a single parameter of type
+   *  Object[].
+   *  The limit 22 is chosen for Scala2x interop. It could be something
+   *  else without affecting the set of programs that can be compiled.
+   */
+  val MaxImplementedFunctionArity = 22
 }
 
 /** A class defining symbols and types of standard definitions
@@ -45,32 +52,29 @@ class Definitions {
     ctx.newSymbol(owner, name, flags | Permanent, info)
 
   private def newClassSymbol(owner: Symbol, name: TypeName, flags: FlagSet, infoFn: ClassSymbol => Type) =
-    ctx.newClassSymbol(owner, name, flags | Permanent, infoFn).entered
+    ctx.newClassSymbol(owner, name, flags | Permanent, infoFn)
 
-  private def newCompleteClassSymbol(owner: Symbol, name: TypeName, flags: FlagSet, parents: List[TypeRef], decls: Scope = newScope) =
+  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 newTopClassSymbol(name: TypeName, flags: FlagSet, parents: List[TypeRef]) =
-    completeClass(newCompleteClassSymbol(ScalaPackageClass, name, flags, parents))
-
-  private def newTypeField(cls: ClassSymbol, name: TypeName, flags: FlagSet, scope: MutableScope) =
+  private def enterTypeField(cls: ClassSymbol, name: TypeName, flags: FlagSet, scope: MutableScope) =
     scope.enter(newSymbol(cls, name, flags, TypeBounds.empty))
 
-  private def newTypeParam(cls: ClassSymbol, name: TypeName, flags: FlagSet, scope: MutableScope) =
-    newTypeField(cls, name, flags | ClassTypeParamCreationFlags, scope)
+  private def enterTypeParam(cls: ClassSymbol, name: TypeName, flags: FlagSet, scope: MutableScope) =
+    enterTypeField(cls, name, flags | ClassTypeParamCreationFlags, scope)
 
-  private def newSyntheticTypeParam(cls: ClassSymbol, scope: MutableScope, paramFlags: FlagSet, suffix: String = "T0") =
-    newTypeParam(cls, suffix.toTypeName.expandedName(cls), ExpandedName | paramFlags, scope)
+  private def enterSyntheticTypeParam(cls: ClassSymbol, paramFlags: FlagSet, scope: MutableScope, suffix: String = "T0") =
+    enterTypeParam(cls, suffix.toTypeName.expandedName(cls), ExpandedName | paramFlags, scope)
 
   // NOTE: Ideally we would write `parentConstrs: => Type*` but SIP-24 is only
   // implemented in Dotty and not in Scala 2.
   // See <http://docs.scala-lang.org/sips/pending/repeated-byname.html>.
-  private def specialPolyClass(name: TypeName, paramFlags: FlagSet, parentConstrs: => Seq[Type]): ClassSymbol = {
+  private def enterSpecialPolyClass(name: TypeName, paramFlags: FlagSet, parentConstrs: => Seq[Type]): ClassSymbol = {
     val completer = new LazyType {
       def complete(denot: SymDenotation)(implicit ctx: Context): Unit = {
         val cls = denot.asClass.classSymbol
         val paramDecls = newScope
-        val typeParam = newSyntheticTypeParam(cls, paramDecls, paramFlags)
+        val typeParam = enterSyntheticTypeParam(cls, paramFlags, paramDecls)
         def instantiate(tpe: Type) =
           if (tpe.typeParams.nonEmpty) tpe.appliedTo(typeParam.typeRef)
           else tpe
@@ -79,31 +83,54 @@ class Definitions {
         denot.info = ClassInfo(ScalaPackageClass.thisType, cls, parentRefs, paramDecls)
       }
     }
-    newClassSymbol(ScalaPackageClass, name, EmptyFlags, completer)
+    newClassSymbol(ScalaPackageClass, name, EmptyFlags, completer).entered
+  }
+
+  /** The trait FunctionN, for some N */
+  private def newFunctionNTrait(n: Int) = {
+    val completer = new LazyType {
+      def complete(denot: SymDenotation)(implicit ctx: Context): Unit = {
+        val cls = denot.asClass.classSymbol
+        val decls = newScope
+        val argParams =
+          for (i <- List.range(0, n)) yield
+            enterTypeParam(cls, s"T$i".toTypeName, Contravariant, decls)
+        val resParam = enterTypeParam(cls, s"R".toTypeName, Covariant, decls)
+        val applyMeth =
+          decls.enter(
+            newMethod(cls, nme.apply,
+              MethodType(argParams.map(_.typeRef), resParam.typeRef), Deferred))
+        denot.info = ClassInfo(ScalaPackageClass.thisType, cls, ObjectType :: Nil, decls)
+      }
+    }
+    newClassSymbol(ScalaPackageClass, s"Function$n".toTypeName, Trait, completer)
   }
 
   private def newMethod(cls: ClassSymbol, name: TermName, info: Type, flags: FlagSet = EmptyFlags): TermSymbol =
-    newSymbol(cls, name.encode, flags | Method, info).entered.asTerm
+    newSymbol(cls, name.encode, flags | Method, info).asTerm
 
-  private def newAliasType(name: TypeName, tpe: Type, flags: FlagSet = EmptyFlags): TypeSymbol = {
+  private def enterMethod(cls: ClassSymbol, name: TermName, info: Type, flags: FlagSet = EmptyFlags): TermSymbol =
+    newMethod(cls, name, info, flags).entered
+
+  private def enterAliasType(name: TypeName, tpe: Type, flags: FlagSet = EmptyFlags): TypeSymbol = {
     val sym = newSymbol(ScalaPackageClass, name, flags, TypeAlias(tpe))
     ScalaPackageClass.currentPackageDecls.enter(sym)
     sym
   }
 
-  private def newPolyMethod(cls: ClassSymbol, name: TermName, typeParamCount: Int,
+  private def enterPolyMethod(cls: ClassSymbol, name: TermName, typeParamCount: Int,
                     resultTypeFn: PolyType => Type, flags: FlagSet = EmptyFlags) = {
     val tparamNames = tpnme.syntheticTypeParamNames(typeParamCount)
     val tparamBounds = tparamNames map (_ => TypeBounds.empty)
     val ptype = PolyType(tparamNames)(_ => tparamBounds, resultTypeFn)
-    newMethod(cls, name, ptype, flags)
+    enterMethod(cls, name, ptype, flags)
   }
 
-  private def newT1ParameterlessMethod(cls: ClassSymbol, name: TermName, resultTypeFn: PolyType => Type, flags: FlagSet) =
-    newPolyMethod(cls, name, 1, resultTypeFn, flags)
+  private def enterT1ParameterlessMethod(cls: ClassSymbol, name: TermName, resultTypeFn: PolyType => Type, flags: FlagSet) =
+    enterPolyMethod(cls, name, 1, resultTypeFn, flags)
 
-  private def newT1EmptyParamsMethod(cls: ClassSymbol, name: TermName, resultTypeFn: PolyType => Type, flags: FlagSet) =
-    newPolyMethod(cls, name, 1, pt => MethodType(Nil, resultTypeFn(pt)), flags)
+  private def enterT1EmptyParamsMethod(cls: ClassSymbol, name: TermName, resultTypeFn: PolyType => Type, flags: FlagSet) =
+    enterPolyMethod(cls, name, 1, pt => MethodType(Nil, resultTypeFn(pt)), flags)
 
   private def mkArityArray(name: String, arity: Int, countFrom: Int): Array[TypeRef] = {
     val arr = new Array[TypeRef](arity + 1)
@@ -172,20 +199,20 @@ class Definitions {
    *       def getClass: java.lang.Class[T] = ???
    *     }
    */
-  lazy val AnyClass: ClassSymbol = completeClass(newCompleteClassSymbol(ScalaPackageClass, tpnme.Any, Abstract, Nil))
+  lazy val AnyClass: ClassSymbol = completeClass(enterCompleteClassSymbol(ScalaPackageClass, tpnme.Any, Abstract, Nil))
   def AnyType = AnyClass.typeRef
-  lazy val AnyValClass: ClassSymbol = completeClass(newCompleteClassSymbol(ScalaPackageClass, tpnme.AnyVal, Abstract, List(AnyClass.typeRef)))
+  lazy val AnyValClass: ClassSymbol = completeClass(enterCompleteClassSymbol(ScalaPackageClass, tpnme.AnyVal, Abstract, List(AnyClass.typeRef)))
   def AnyValType = AnyValClass.typeRef
 
-    lazy val Any_==       = newMethod(AnyClass, nme.EQ, methOfAny(BooleanType), Final)
-    lazy val Any_!=       = newMethod(AnyClass, nme.NE, methOfAny(BooleanType), Final)
-    lazy val Any_equals   = newMethod(AnyClass, nme.equals_, methOfAny(BooleanType))
-    lazy val Any_hashCode = newMethod(AnyClass, nme.hashCode_, MethodType(Nil, IntType))
-    lazy val Any_toString = newMethod(AnyClass, nme.toString_, MethodType(Nil, StringType))
-    lazy val Any_##       = newMethod(AnyClass, nme.HASHHASH, ExprType(IntType), Final)
-    lazy val Any_getClass = newMethod(AnyClass, nme.getClass_, MethodType(Nil, ClassClass.typeRef.appliedTo(TypeBounds.empty)), Final)
-    lazy val Any_isInstanceOf = newT1ParameterlessMethod(AnyClass, nme.isInstanceOf_, _ => BooleanType, Final)
-    lazy val Any_asInstanceOf = newT1ParameterlessMethod(AnyClass, nme.asInstanceOf_, PolyParam(_, 0), Final)
+    lazy val Any_==       = enterMethod(AnyClass, nme.EQ, methOfAny(BooleanType), Final)
+    lazy val Any_!=       = enterMethod(AnyClass, nme.NE, methOfAny(BooleanType), Final)
+    lazy val Any_equals   = enterMethod(AnyClass, nme.equals_, methOfAny(BooleanType))
+    lazy val Any_hashCode = enterMethod(AnyClass, nme.hashCode_, MethodType(Nil, IntType))
+    lazy val Any_toString = enterMethod(AnyClass, nme.toString_, MethodType(Nil, StringType))
+    lazy val Any_##       = enterMethod(AnyClass, nme.HASHHASH, ExprType(IntType), Final)
+    lazy val Any_getClass = enterMethod(AnyClass, nme.getClass_, MethodType(Nil, ClassClass.typeRef.appliedTo(TypeBounds.empty)), Final)
+    lazy val Any_isInstanceOf = enterT1ParameterlessMethod(AnyClass, nme.isInstanceOf_, _ => BooleanType, Final)
+    lazy val Any_asInstanceOf = enterT1ParameterlessMethod(AnyClass, nme.asInstanceOf_, PolyParam(_, 0), Final)
 
     def AnyMethods = List(Any_==, Any_!=, Any_equals, Any_hashCode,
       Any_toString, Any_##, Any_getClass, Any_isInstanceOf, Any_asInstanceOf)
@@ -205,37 +232,37 @@ class Definitions {
   }
   def ObjectType = ObjectClass.typeRef
 
-  lazy val AnyRefAlias: TypeSymbol = newAliasType(tpnme.AnyRef, ObjectType)
+  lazy val AnyRefAlias: TypeSymbol = enterAliasType(tpnme.AnyRef, ObjectType)
   def AnyRefType = AnyRefAlias.typeRef
 
-    lazy val Object_eq = newMethod(ObjectClass, nme.eq, methOfAnyRef(BooleanType), Final)
-    lazy val Object_ne = newMethod(ObjectClass, nme.ne, methOfAnyRef(BooleanType), Final)
-    lazy val Object_synchronized = newPolyMethod(ObjectClass, nme.synchronized_, 1,
+    lazy val Object_eq = enterMethod(ObjectClass, nme.eq, methOfAnyRef(BooleanType), Final)
+    lazy val Object_ne = enterMethod(ObjectClass, nme.ne, methOfAnyRef(BooleanType), Final)
+    lazy val Object_synchronized = enterPolyMethod(ObjectClass, nme.synchronized_, 1,
         pt => MethodType(List(PolyParam(pt, 0)), PolyParam(pt, 0)), Final)
-    lazy val Object_clone = newMethod(ObjectClass, nme.clone_, MethodType(Nil, ObjectType), Protected)
-    lazy val Object_finalize = newMethod(ObjectClass, nme.finalize_, MethodType(Nil, UnitType), Protected)
-    lazy val Object_notify = newMethod(ObjectClass, nme.notify_, MethodType(Nil, UnitType))
-    lazy val Object_notifyAll = newMethod(ObjectClass, nme.notifyAll_, MethodType(Nil, UnitType))
-    lazy val Object_wait = newMethod(ObjectClass, nme.wait_, MethodType(Nil, UnitType))
-    lazy val Object_waitL = newMethod(ObjectClass, nme.wait_, MethodType(LongType :: Nil, UnitType))
-    lazy val Object_waitLI = newMethod(ObjectClass, nme.wait_, MethodType(LongType :: IntType :: Nil, UnitType))
+    lazy val Object_clone = enterMethod(ObjectClass, nme.clone_, MethodType(Nil, ObjectType), Protected)
+    lazy val Object_finalize = enterMethod(ObjectClass, nme.finalize_, MethodType(Nil, UnitType), Protected)
+    lazy val Object_notify = enterMethod(ObjectClass, nme.notify_, MethodType(Nil, UnitType))
+    lazy val Object_notifyAll = enterMethod(ObjectClass, nme.notifyAll_, MethodType(Nil, UnitType))
+    lazy val Object_wait = enterMethod(ObjectClass, nme.wait_, MethodType(Nil, UnitType))
+    lazy val Object_waitL = enterMethod(ObjectClass, nme.wait_, MethodType(LongType :: Nil, UnitType))
+    lazy val Object_waitLI = enterMethod(ObjectClass, nme.wait_, MethodType(LongType :: IntType :: Nil, UnitType))
 
     def ObjectMethods = List(Object_eq, Object_ne, Object_synchronized, Object_clone,
         Object_finalize, Object_notify, Object_notifyAll, Object_wait, Object_waitL, Object_waitLI)
 
   /** Dummy method needed by elimByName */
-  lazy val dummyApply = newPolyMethod(
+  lazy val dummyApply = enterPolyMethod(
       OpsPackageClass, nme.dummyApply, 1,
       pt => MethodType(List(FunctionOf(Nil, PolyParam(pt, 0))), PolyParam(pt, 0)))
 
   /** Method representing a throw */
-  lazy val throwMethod = newMethod(OpsPackageClass, nme.THROWkw,
+  lazy val throwMethod = enterMethod(OpsPackageClass, nme.THROWkw,
       MethodType(List(ThrowableType), NothingType))
 
-  lazy val NothingClass: ClassSymbol = newCompleteClassSymbol(
+  lazy val NothingClass: ClassSymbol = enterCompleteClassSymbol(
     ScalaPackageClass, tpnme.Nothing, AbstractFinal, List(AnyClass.typeRef))
   def NothingType = NothingClass.typeRef
-  lazy val NullClass: ClassSymbol = newCompleteClassSymbol(
+  lazy val NullClass: ClassSymbol = enterCompleteClassSymbol(
     ScalaPackageClass, tpnme.Null, AbstractFinal, List(ObjectClass.typeRef))
   def NullType = NullClass.typeRef
 
@@ -281,7 +308,7 @@ class Definitions {
   lazy val SingletonClass: ClassSymbol =
     // needed as a synthetic class because Scala 2.x refers to it in classfiles
     // but does not define it as an explicit class.
-    newCompleteClassSymbol(
+    enterCompleteClassSymbol(
       ScalaPackageClass, tpnme.Singleton, PureInterfaceCreationFlags | Final,
       List(AnyClass.typeRef), EmptyScope)
 
@@ -387,17 +414,17 @@ class Definitions {
   lazy val BoxedDoubleModule  = ctx.requiredModule("java.lang.Double")
   lazy val BoxedUnitModule    = ctx.requiredModule("java.lang.Void")
 
-  lazy val ByNameParamClass2x = specialPolyClass(tpnme.BYNAME_PARAM_CLASS, Covariant, Seq(AnyType))
-  lazy val EqualsPatternClass = specialPolyClass(tpnme.EQUALS_PATTERN, EmptyFlags, Seq(AnyType))
+  lazy val ByNameParamClass2x = enterSpecialPolyClass(tpnme.BYNAME_PARAM_CLASS, Covariant, Seq(AnyType))
+  lazy val EqualsPatternClass = enterSpecialPolyClass(tpnme.EQUALS_PATTERN, EmptyFlags, Seq(AnyType))
 
-  lazy val RepeatedParamClass = specialPolyClass(tpnme.REPEATED_PARAM_CLASS, Covariant, Seq(ObjectType, SeqType))
+  lazy val RepeatedParamClass = enterSpecialPolyClass(tpnme.REPEATED_PARAM_CLASS, Covariant, Seq(ObjectType, SeqType))
 
   // fundamental classes
   lazy val StringClass                = ctx.requiredClass("java.lang.String")
   def StringType: Type                = StringClass.typeRef
   lazy val StringModule               = StringClass.linkedClass
 
-    lazy val String_+ = newMethod(StringClass, nme.raw.PLUS, methOfAny(StringType), Final)
+    lazy val String_+ = enterMethod(StringClass, nme.raw.PLUS, methOfAny(StringType), Final)
     lazy val String_valueOf_Object = StringModule.info.member(nme.valueOf).suchThat(_.info.firstParamTypes match {
       case List(pt) => (pt isRef AnyClass) || (pt isRef ObjectClass)
       case _ => false
@@ -431,6 +458,9 @@ class Definitions {
   def PartialFunctionClass(implicit ctx: Context) = PartialFunctionType.symbol.asClass
   lazy val AbstractPartialFunctionType: TypeRef = ctx.requiredClassRef("scala.runtime.AbstractPartialFunction")
   def AbstractPartialFunctionClass(implicit ctx: Context) = AbstractPartialFunctionType.symbol.asClass
+  lazy val FunctionXXLType: TypeRef         = ctx.requiredClassRef("scala.FunctionXXL")
+  def FunctionXXLClass(implicit ctx: Context) = FunctionXXLType.symbol.asClass
+
   lazy val SymbolType: TypeRef                  = ctx.requiredClassRef("scala.Symbol")
   def SymbolClass(implicit ctx: Context) = SymbolType.symbol.asClass
   lazy val DynamicType: TypeRef                 = ctx.requiredClassRef("scala.Dynamic")
@@ -562,14 +592,15 @@ class Definitions {
   object FunctionOf {
     def apply(args: List[Type], resultType: Type)(implicit ctx: Context) =
       FunctionType(args.length).appliedTo(args ::: resultType :: Nil)
-    def unapply(ft: Type)(implicit ctx: Context)/*: Option[(List[Type], Type)]*/ = {
-      // -language:keepUnions difference: unapply needs result type because inferred type
-      // is Some[(List[Type], Type)] | None, which is not a legal unapply type.
+    def unapply(ft: Type)(implicit ctx: Context) = {
       val tsym = ft.typeSymbol
-      lazy val targs = ft.argInfos
-      val numArgs = targs.length - 1
-      if (numArgs >= 0 && numArgs <= MaxFunctionArity &&
-          (FunctionType(numArgs).symbol == tsym)) Some(targs.init, targs.last)
+      if (isFunctionClass(tsym)) {
+        lazy val targs = ft.argInfos
+        val numArgs = targs.length - 1
+        if (numArgs >= 0 && FunctionType(numArgs).symbol == tsym)
+          Some(targs.init, targs.last)
+        else None
+      }
       else None
     }
   }
@@ -612,19 +643,26 @@ class Definitions {
 
   // ----- Symbol sets ---------------------------------------------------
 
-  lazy val AbstractFunctionType = mkArityArray("scala.runtime.AbstractFunction", MaxAbstractFunctionArity, 0)
+  lazy val AbstractFunctionType = mkArityArray("scala.runtime.AbstractFunction", MaxImplementedFunctionArity, 0)
   val AbstractFunctionClassPerRun = new PerRun[Array[Symbol]](implicit ctx => AbstractFunctionType.map(_.symbol.asClass))
   def AbstractFunctionClass(n: Int)(implicit ctx: Context) = AbstractFunctionClassPerRun()(ctx)(n)
-  lazy val FunctionType = mkArityArray("scala.Function", MaxFunctionArity, 0)
-  def FunctionClassPerRun = new PerRun[Array[Symbol]](implicit ctx => FunctionType.map(_.symbol.asClass))
-  def FunctionClass(n: Int)(implicit ctx: Context) = FunctionClassPerRun()(ctx)(n)
-    lazy val Function0_applyR = FunctionType(0).symbol.requiredMethodRef(nme.apply)
-    def Function0_apply(implicit ctx: Context) = Function0_applyR.symbol
+  private lazy val ImplementedFunctionType = mkArityArray("scala.Function", MaxImplementedFunctionArity, 0)
+  def FunctionClassPerRun = new PerRun[Array[Symbol]](implicit ctx => ImplementedFunctionType.map(_.symbol.asClass))
 
   lazy val TupleType = mkArityArray("scala.Tuple", MaxTupleArity, 2)
   lazy val ProductNType = mkArityArray("scala.Product", MaxTupleArity, 0)
 
-  private lazy val FunctionTypes: Set[TypeRef] = FunctionType.toSet
+  def FunctionClass(n: Int)(implicit ctx: Context) =
+    if (n < MaxImplementedFunctionArity) FunctionClassPerRun()(ctx)(n)
+    else ctx.requiredClass("scala.Function" + n.toString)
+
+    lazy val Function0_applyR = ImplementedFunctionType(0).symbol.requiredMethodRef(nme.apply)
+    def Function0_apply(implicit ctx: Context) = Function0_applyR.symbol
+
+  def FunctionType(n: Int)(implicit ctx: Context): TypeRef =
+    if (n < MaxImplementedFunctionArity) ImplementedFunctionType(n)
+    else FunctionClass(n).typeRef
+
   private lazy val TupleTypes: Set[TypeRef] = TupleType.toSet
   private lazy val ProductTypes: Set[TypeRef] = ProductNType.toSet
 
@@ -646,6 +684,8 @@ class Definitions {
     tp.derivesFrom(NothingClass) || tp.derivesFrom(NullClass)
 
   def isFunctionClass(cls: Symbol) = isVarArityClass(cls, tpnme.Function)
+  def isUnimplementedFunctionClass(cls: Symbol) =
+    isFunctionClass(cls) && cls.name.functionArity > MaxImplementedFunctionArity
   def isAbstractFunctionClass(cls: Symbol) = isVarArityClass(cls, tpnme.AbstractFunction)
   def isTupleClass(cls: Symbol) = isVarArityClass(cls, tpnme.Tuple)
   def isProductClass(cls: Symbol) = isVarArityClass(cls, tpnme.Product)
@@ -695,10 +735,11 @@ class Definitions {
   def isProductSubType(tp: Type)(implicit ctx: Context) =
     (tp derivesFrom ProductType.symbol) && tp.baseClasses.exists(isProductClass)
 
-  def isFunctionType(tp: Type)(implicit ctx: Context) = {
-    val arity = functionArity(tp)
-    0 <= arity && arity <= MaxFunctionArity && (tp isRef FunctionType(arity).symbol)
-  }
+  def isFunctionType(tp: Type)(implicit ctx: Context) =
+    isFunctionClass(tp.dealias.typeSymbol) && {
+      val arity = functionArity(tp)
+      arity >= 0 && tp.isRef(FunctionType(functionArity(tp)).typeSymbol)
+    }
 
   def functionArity(tp: Type)(implicit ctx: Context) = tp.dealias.argInfos.length - 1
 
@@ -774,6 +815,23 @@ class Definitions {
 
   // ----- Initialization ---------------------------------------------------
 
+  /** Give the scala package a scope where a FunctionN trait is automatically
+   *  added when someone looks for it.
+   */
+  private def makeScalaSpecial()(implicit ctx: Context) = {
+    val oldInfo = ScalaPackageClass.classInfo
+    val oldDecls = oldInfo.decls
+    val newDecls = new MutableScope(oldDecls) {
+      override def lookupEntry(name: Name)(implicit ctx: Context): ScopeEntry = {
+        val res = super.lookupEntry(name)
+        if (res == null && name.functionArity > 0)
+          newScopeEntry(newFunctionNTrait(name.functionArity))
+        else res
+      }
+    }
+    ScalaPackageClass.info = oldInfo.derivedClassInfo(decls = newDecls)
+  }
+
   /** Lists core classes that don't have underlying bytecode, but are synthesized on-the-fly in every reflection universe */
   lazy val syntheticScalaClasses = List(
     AnyClass,
@@ -801,6 +859,8 @@ class Definitions {
   def init()(implicit ctx: Context) = {
     this.ctx = ctx
     if (!_isInitialized) {
+      makeScalaSpecial()
+
       // force initialization of every symbol that is synthesized or hijacked by the compiler
       val forced = syntheticCoreClasses ++ syntheticCoreMethods ++ ScalaValueClasses()