Use backend magic to handle super calls

The problem can be seen from the following example:

trait A { def foo() = ??? }
trait B { def foo() = ??? }

object C extends A with B {
	super[A].foo()
	super[B].foo()
}

In the code above, we cannot translate the following calls
from <init> to <clinit>:

  super[A].foo()
  super[B].foo()

  super[A].$iinit$()
  super[B].$init$()

More details can be found here: #5928

A principled way would be to generage super accessors as it
is done in posttyper.

However, the backend has a magic to support
prefix to super trees in [1], which is exploited
in the Scala 2 fix [2].

[1] scala/scala#5944
[2] scala/scala#7270

Author: liufengyun

compiler/src/dotty/tools/backend/jvm/BCodeBodyBuilder.scala

@@ -700,14 +700,16 @@ trait BCodeBodyBuilder extends BCodeSkelBuilder {
             if (t.symbol ne defn.Object_synchronized) genTypeApply(t)
             else genSynchronized(app, expectedType)
 
-        case Apply(fun @ DesugaredSelect(Super(_, _), _), args) =>
+        case Apply(fun @ DesugaredSelect(Super(superQual, _), _), args) =>
           // 'super' call: Note: since constructors are supposed to
           // return an instance of what they construct, we have to take
           // special care. On JVM they are 'void', and Scala forbids (syntactically)
           // to call super constructors explicitly and/or use their 'returned' value.
           // therefore, we can ignore this fact, and generate code that leaves nothing
           // on the stack (contrary to what the type in the AST says).
-          mnode.visitVarInsn(asm.Opcodes.ALOAD, 0)
+
+          // scala/bug#10290: qual can be `this.$outer()` (not just `this`), so we call genLoad (not just ALOAD_0)
+          genLoad(superQual)
           genLoadArguments(args, paramTKs(app))
           generatedType = genCallMethod(fun.symbol, InvokeStyle.Super, app.span)
 

compiler/src/dotty/tools/backend/jvm/BCodeSkelBuilder.scala

@@ -156,10 +156,6 @@ trait BCodeSkelBuilder extends BCodeHelpers {
             tree.withType(tp) match {
               case tree: This if tree.symbol == claszSymbol =>
                 ref(claszSymbol.sourceModule)
-            case Apply(fun @ Select(Super(qual, _), _), args) if qual.symbol == claszSymbol =>
-              ref(claszSymbol.sourceModule).select(fun.symbol).appliedToArgs(args)
-              // case ident: Ident =>
-              //   super.transform(desugarIdent(ident))
               case tree =>
                 super.transform(tree)
             }