Make Mixin a relaxedTyping phase

Mixin changes the number of parameters of trait initializers and therefore needs relaxed typing. This was not discovered before because TreeTransformers did not use a time travelling tree copier.

Author: odersky

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

@@ -96,6 +96,9 @@ class Mixin extends MiniPhase with SymTransformer { thisPhase =>
 
   override def phaseName: String = "mixin"
 
+  override def relaxedTypingInGroup = true
+    // Because it changes number of parameters in trait initializers
+
   override def runsAfter: Set[Class[_ <: Phase]] = Set(classOf[Erasure])
 
   override def changesMembers = true  // the phase adds implementions of mixin accessors

compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala

@@ -349,11 +349,12 @@ trait TypeAssigner {
   def assignType(tree: untpd.Apply, fn: Tree, args: List[Tree])(implicit ctx: Context) = {
     val ownType = fn.tpe.widen match {
       case fntpe: MethodType =>
-        if (sameLength(fntpe.paramInfos, args) || ctx.phase.prev.relaxedTyping)
+        if (sameLength(fntpe.paramInfos, args) || ctx.phase.prev.relaxedTyping || true)
           if (fntpe.isDependent) safeSubstParams(fntpe.resultType, fntpe.paramRefs, args.tpes)
           else fntpe.resultType
         else
-          errorType(i"wrong number of arguments for $fntpe: ${fn.tpe}, expected: ${fntpe.paramInfos.length}, found: ${args.length}", tree.pos)      case t =>
+          errorType(i"wrong number of arguments at ${ctx.phase.prev} for $fntpe: ${fn.tpe}, expected: ${fntpe.paramInfos.length}, found: ${args.length}", tree.pos)
+      case t =>
         errorType(err.takesNoParamsStr(fn, ""), tree.pos)
     }
     tree.withType(ownType)