Drop function 22 limit.

Functions with more than 22 parameters are now
automatically converted to functions taking
a single object array parameter.

This has been achieved by tweaking erasure.

Other things I have tried that did ot work out well:

 - Use a single function type in typer. The problem with this
   one which could not be circumvented was that existing higher-kinded
   code with e.g. Funcor assumes that Functon1 is a binary type constructor.

 - Have a late phase that converts to FunctonXXL instead of
   doing it in erasure. The problem with that one was that
   potentially every type could be affected, which was ill-suited
   to the architecture of a miniphase.

3 files changed, 24 insertions, 7 deletions

diff --git a/compiler/src/dotty/tools/dotc/core/Definitions.scala b/compiler/src/dotty/tools/dotc/core/Definitions.scala
index e1a87d1f7..29b7bb60a 100644
--- a/compiler/src/dotty/tools/dotc/core/Definitions.scala
+++ b/compiler/src/dotty/tools/dotc/core/Definitions.scala
@@ -457,6 +457,9 @@ 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")
@@ -645,6 +648,9 @@ class Definitions {
   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)
+
   def FunctionClass(n: Int)(implicit ctx: Context) =
     if (n < MaxImplementedFunctionArity) FunctionClassPerRun()(ctx)(n)
     else ctx.requiredClass("scala.Function" + n.toString)
@@ -652,10 +658,7 @@ class Definitions {
     lazy val Function0_applyR = ImplementedFunctionType(0).symbol.requiredMethodRef(nme.apply)
     def Function0_apply(implicit ctx: Context) = Function0_applyR.symbol
 
-  lazy val TupleType = mkArityArray("scala.Tuple", MaxTupleArity, 2)
-  lazy val ProductNType = mkArityArray("scala.Product", MaxTupleArity, 0)
-
-  def FunctionType(n: Int): TypeRef =
+  def FunctionType(n: Int)(implicit ctx: Context): TypeRef =
     if (n < MaxImplementedFunctionArity) ImplementedFunctionType(n)
     else FunctionClass(n).typeRef
 
@@ -680,6 +683,8 @@ 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)
diff --git a/compiler/src/dotty/tools/dotc/core/StdNames.scala b/compiler/src/dotty/tools/dotc/core/StdNames.scala
index c2a14b36f..741ff8b1f 100644
--- a/compiler/src/dotty/tools/dotc/core/StdNames.scala
+++ b/compiler/src/dotty/tools/dotc/core/StdNames.scala
@@ -265,6 +265,7 @@ object StdNames {
     val THIS: N                     = "_$this"
     val TRAIT_CONSTRUCTOR: N        = "$init$"
     val U2EVT: N                    = "u2evt$"
+    val ALLARGS: N                  = "$allArgs"
 
     final val Nil: N                = "Nil"
     final val Predef: N             = "Predef"
diff --git a/compiler/src/dotty/tools/dotc/core/TypeErasure.scala b/compiler/src/dotty/tools/dotc/core/TypeErasure.scala
index abbacee49..57397a8bc 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeErasure.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeErasure.scala
@@ -7,6 +7,7 @@ import Uniques.unique
 import dotc.transform.ExplicitOuter._
 import dotc.transform.ValueClasses._
 import util.DotClass
+import Definitions.MaxImplementedFunctionArity
 
 /** Erased types are:
  *
@@ -38,7 +39,10 @@ object TypeErasure {
     case _: ErasedValueType =>
       true
     case tp: TypeRef =>
-      tp.symbol.isClass && tp.symbol != defn.AnyClass && tp.symbol != defn.ArrayClass
+      val sym = tp.symbol
+      sym.isClass &&
+      sym != defn.AnyClass && sym != defn.ArrayClass &&
+      !defn.isUnimplementedFunctionClass(sym)
     case _: TermRef =>
       true
     case JavaArrayType(elem) =>
@@ -176,8 +180,13 @@ object TypeErasure {
     else if (sym.isAbstractType) TypeAlias(WildcardType)
     else if (sym.isConstructor) outer.addParam(sym.owner.asClass, erase(tp)(erasureCtx))
     else erase.eraseInfo(tp, sym)(erasureCtx) match {
-      case einfo: MethodType if sym.isGetter && einfo.resultType.isRef(defn.UnitClass) =>
-        MethodType(Nil, defn.BoxedUnitType)
+      case einfo: MethodType =>
+        if (sym.isGetter && einfo.resultType.isRef(defn.UnitClass))
+          MethodType(Nil, defn.BoxedUnitType)
+        else if (sym.isAnonymousFunction && einfo.paramTypes.length > MaxImplementedFunctionArity)
+          MethodType(nme.ALLARGS :: Nil, JavaArrayType(defn.ObjectType) :: Nil, einfo.resultType)
+        else
+          einfo
       case einfo =>
         einfo
     }
@@ -317,6 +326,7 @@ class TypeErasure(isJava: Boolean, semiEraseVCs: Boolean, isConstructor: Boolean
    *   - For a term ref p.x, the type <noprefix> # x.
    *   - For a typeref scala.Any, scala.AnyVal or scala.Singleton: |java.lang.Object|
    *   - For a typeref scala.Unit, |scala.runtime.BoxedUnit|.
+   *   - For a typeref scala.FunctionN, where N > MaxImplementedFunctionArity, scala.FunctionXXL
    *   - For a typeref P.C where C refers to a class, <noprefix> # C.
    *   - For a typeref P.C where C refers to an alias type, the erasure of C's alias.
    *   - For a typeref P.C where C refers to an abstract type, the erasure of C's upper bound.
@@ -345,6 +355,7 @@ class TypeErasure(isJava: Boolean, semiEraseVCs: Boolean, isConstructor: Boolean
       if (!sym.isClass) this(tp.info)
       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.isUnimplementedFunctionClass(sym)) defn.FunctionXXLType
       else eraseNormalClassRef(tp)
     case tp: RefinedType =>
       val parent = tp.parent