Reorder unpickled type params if necessary

src/dotty/tools/dotc/core/SymDenotations.scala

@@ -176,7 +176,7 @@ object SymDenotations {
       // completions.println(s"completed ${this.debugString}")
     }
 
-    protected[dotc] final def info_=(tp: Type) = {
+    protected[dotc] def info_=(tp: Type) = {
       /* // DEBUG
        def illegal: String = s"illegal type for $this: $tp"
       if (this is Module) // make sure module invariants that allow moduleClass and sourceModule to work are kept.
@@ -1177,6 +1177,11 @@ object SymDenotations {
       myTypeParams
     }
 
+    override protected[dotc] final def info_=(tp: Type) = {
+      super.info_=(tp)
+      myTypeParams = null // changing the info might change decls, and with it typeParams
+    }
+
     /** The denotations of all parents in this class. */
     def classParents(implicit ctx: Context): List[TypeRef] = info match {
       case classInfo: ClassInfo => classInfo.classParents

src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -130,8 +130,18 @@ object Scala2Unpickler {
     } else {
       registerCompanionPair(scalacCompanion, denot.classSymbol)
     }
+    val declsTypeParams = denot.typeParams
+    val declsInRightOrder =
+      if (declsTypeParams.corresponds(tparams)(_.name == _.name)) decls
+      else { // create new scope with type parameters in right order
+        val decls1 = newScope
+        for (tparam <- tparams) decls1.enter(decls.lookup(tparam.name))
+        for (sym <- decls) if (!declsTypeParams.contains(sym)) decls1.enter(sym)
+        decls1
+      }
 
-    denot.info = ClassInfo(denot.owner.thisType, denot.classSymbol, parentRefs, decls, ost)
+    denot.info = ClassInfo(
+      denot.owner.thisType, denot.classSymbol, parentRefs, declsInRightOrder, ost)
   }
 }