Fix 2.12 self types in mixin translation

2.12 seems to always assume the enclsing mixin class as the type of the "$this" self parameter of
a mixin implementation method whereas 2.11 used the self type of this class instead. This commit
compensates or the difference.

Author: odersky

compiler/src/dotty/tools/dotc/transform/LinkScala2Impls.scala

@@ -50,9 +50,18 @@ class LinkScala2Impls extends MiniPhase with IdentityDenotTransformer { thisTran
 
     /** Copy definitions from implementation class to trait itself */
     private def augmentScala_2_12_Trait(mixin: ClassSymbol)(implicit ctx: Context): Unit = {
+      def info_2_12(sym: Symbol) = sym.info match {
+        case mt @ MethodType(paramNames @ nme.SELF :: _) =>
+          // 2.12 seems to always assume the enclsing mixin class as self type parameter,
+          // whereas 2.11 used the self type of this class instead.
+          val selfType :: otherParamTypes = mt.paramInfos
+          MethodType(paramNames, mixin.typeRef :: otherParamTypes, mt.resType)
+        case info => info
+      }
       def newImpl(sym: TermSymbol): Symbol = sym.copy(
         owner = mixin,
-        name = if (sym.isConstructor) sym.name else ImplMethName(sym.name)
+        name = if (sym.isConstructor) sym.name else ImplMethName(sym.name),
+        info = info_2_12(sym)
       )
       for (sym <- mixin.implClass.info.decls)
         newImpl(sym.asTerm).enteredAfter(thisTransform)

tests/run/mixins1/A_1.scala

@@ -1,11 +1,11 @@
-trait A {
-
+trait A[This <: A[This] with B] { self: B =>
   var x = 3
   println("hi")
   val y = x * x
 
   def f: Int = x + y
 
   def f(z: Int): Int = f + z
-
 }
+
+trait B extends A[B]

tests/run/mixins1/C_2.scala

@@ -1,5 +1,5 @@
 // Intended to be compiled with either 2.11 or 2.12
-class C extends A {
+class C extends A with B {
   x = 4
 
   override def f: Int = super.f
@@ -8,5 +8,5 @@ class C extends A {
 }
 
 object Test extends App {
-  new C().f
+  println(new C().f)
 }