Keep constructor proxies after erasure

It's awkward to delete all references to them. We should treat them like other
symbols that are effectively erased.

Author: odersky

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

@@ -214,7 +214,7 @@ class BTypesFromSymbols[I <: DottyBackendInterface](val int: I) extends BTypes {
   private def definedClasses(sym: Symbol, phase: Phase) =
     if (sym.isDefinedInCurrentRun)
       atPhase(phase) {
-        toDenot(sym).info.decls.filter(_.isClass)
+        toDenot(sym).info.decls.filter(sym => sym.isClass && !sym.isEffectivelyErased)
       }
     else Nil
 

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

@@ -1048,8 +1048,6 @@ object Erasure {
           cls.linkedClass.copySymDenotation(initFlags = EmptyFlags, info = NoType)
             .installAfter(erasurePhase)
         cls.registeredCompanion = NoSymbol
-      for mbr <- cls.info.decls do
-        if mbr.is(ConstructorProxy) then mbr.dropAfter(erasurePhase)
 
     override def typedClassDef(cdef: untpd.TypeDef, cls: ClassSymbol)(using Context): Tree =
       if cls.is(Flags.Erased) then erasedDef(cls)

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

@@ -446,8 +446,7 @@ class TreeChecker extends Phase with SymTransformer {
       val decls   = cls.classInfo.decls.toList.toSet.filter(isNonMagicalMember)
       val defined = impl.body.map(_.symbol)
 
-      def isAllowed(sym: Symbol): Boolean =
-        sym.is(ConstructorProxy) && !ctx.phase.erasedTypes
+      def isAllowed(sym: Symbol): Boolean = sym.is(ConstructorProxy)
 
       val symbolsNotDefined = (decls -- defined - constr.symbol).filterNot(isAllowed)