Create FunctionN types on demand

We know create FunctionN types on demand whenever their name
is looked up in the scope of package `scala`. This obviates
the need to predefine function traits 23 to 30.

Author: odersky

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

@@ -767,6 +767,23 @@ 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,
@@ -794,6 +811,8 @@ 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()
 

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

@@ -229,6 +229,12 @@ object NameOps {
       }
     }
 
+    def functionArity: Int =
+      if (name.startsWith(tpnme.Function))
+        try name.drop(tpnme.Function.length).toString.toInt
+        catch { case ex: NumberFormatException => -1 }
+      else -1
+
     /** The name of the generic runtime operation corresponding to an array operation */
     def genericArrayOp: TermName = name match {
       case nme.apply => nme.array_apply

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

@@ -309,7 +309,7 @@ object Scopes {
 
     /** Lookup a symbol entry matching given name.
      */
-    override final def lookupEntry(name: Name)(implicit ctx: Context): ScopeEntry = {
+    override def lookupEntry(name: Name)(implicit ctx: Context): ScopeEntry = {
       var e: ScopeEntry = null
       if (hashTable ne null) {
         e = hashTable(name.hashCode & (hashTable.length - 1))