Whitelist some Scala-2 classes as pure

Currently, this is not exploited but some rewrites might become more effective that way in the future.

Author: odersky

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

@@ -1137,6 +1137,10 @@ class Definitions {
   def isErasedFunctionType(tp: Type)(implicit ctx: Context) =
     isFunctionType(tp) && tp.dealias.typeSymbol.name.isErasedFunction
 
+  /** A whitelist of Scala-2 classes that are known to be pure */
+  def isAssuredNoInits(sym: Symbol) =
+    (sym `eq` SomeClass) || isTupleClass(sym)
+
   // ----- primitive value class machinery ------------------------------------------
 
   /** This class would also be obviated by the implicit function type design */

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

@@ -596,7 +596,8 @@ object SymDenotations {
      *  initaliazion code?
      */
     def isNoInitsClass(implicit ctx: Context) =
-      isClass && asClass.baseClasses.forall(_.is(NoInits))
+      isClass &&
+      (asClass.baseClasses.forall(_.is(NoInits)) || defn.isAssuredNoInits(symbol))
 
     /** Is this a "real" method? A real method is a method which is:
      *  - not an accessor