Fix infinite recursion when creating extension methods

Phase ExtensionMethods creates new symbols for extension methods
and then installs these symbols into the companion object of a value class.
It's important that the creation of these symbols is done in the phase
ExtensionMethods itself, and not in the next phase, as was done before.
If we do it in the next phase, we need the owner at the next phase
and with the new scheme of computeDenot that owner might be forced,
leading to an infinite cycle.

Author: odersky

src/dotty/tools/dotc/transform/ExtensionMethods.scala

@@ -54,7 +54,7 @@ class ExtensionMethods extends MiniPhaseTransform with DenotTransformer with Ful
           ctx.atPhase(thisTransformer.next) { implicit ctx =>
             // In Scala 2, extension methods are added before pickling so we should
             // not generate them again.
-            if (!(origClass is Scala2x)) {
+            if (!(origClass is Scala2x)) ctx.atPhase(thisTransformer) { implicit ctx =>
               for (decl <- origClass.classInfo.decls) {
                 if (isMethodWithExtension(decl))
                   decls1.enter(createExtensionMethod(decl, ref.symbol))
@@ -91,7 +91,6 @@ class ExtensionMethods extends MiniPhaseTransform with DenotTransformer with Ful
     else NoSymbol
 
   private def createExtensionMethod(imeth: Symbol, staticClass: Symbol)(implicit ctx: Context): TermSymbol = {
-    assert(ctx.phase == thisTransformer.next)
     val extensionName = extensionNames(imeth).head.toTermName
     val extensionMeth = ctx.newSymbol(staticClass, extensionName,
       imeth.flags | Final &~ (Override | Protected | AbsOverride),