Always keep annotations on value parameters for class methods

The previous code always dropped annotations from all constructor parameters
and also from all parameters of derived methods such as copy and apply. That
last part was an error: there was code that meant to compensate for it, but
because of a typo the compensation code did nothing. The commit that dropped
parameters was b602d83 "Drop annotations from constructor parameters". That
commit had as motivation that cyclic references could be avoided by dropping
@specialized on type parameters. So by that logic it seems safe to keep annotations
on value parameters of constructors. This could be important if (say) we have an
annotation indicating default arguments, which certainly will have to be kept
on the constructor as well.

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -277,7 +277,7 @@ object desugar {
           DefDef(
             name = DefaultGetterName(methName, n),
             tparams = meth.tparams.map(tparam => dropContextBounds(toDefParam(tparam, keepAnnotations = true))),
-            vparamss = takeUpTo(normalizedVparamss.nestedMap(toDefParam(_, keepAnnotations = true)), n),
+            vparamss = takeUpTo(normalizedVparamss.nestedMap(toDefParam(_)), n),
             tpt = TypeTree(),
             rhs = vparam.rhs
           )
@@ -386,11 +386,8 @@ object desugar {
     if (!keepAnnotations) mods = mods.withAnnotations(Nil)
     tparam.withMods(mods & EmptyFlags | Param)
   }
-  private def toDefParam(vparam: ValDef, keepAnnotations: Boolean): ValDef = {
-    var mods = vparam.rawMods
-    if (!keepAnnotations) mods = mods.withAnnotations(Nil)
-    vparam.withMods(mods & (GivenOrImplicit | Erased) | Param)
-  }
+  private def toDefParam(vparam: ValDef): ValDef =
+    vparam.withMods(vparam.rawMods & (GivenOrImplicit | Erased) | Param)
 
   /** The expansion of a class definition. See inline comments for what is involved */
   def classDef(cdef: TypeDef)(implicit ctx: Context): Tree = {
@@ -463,7 +460,7 @@ object desugar {
         ctx.error(CaseClassMissingNonImplicitParamList(cdef), namePos)
         ListOfNil
       }
-      else originalVparamss.nestedMap(toDefParam(_, keepAnnotations = false))
+      else originalVparamss.nestedMap(toDefParam(_))
     val constr = cpy.DefDef(constr1)(tparams = constrTparams, vparamss = constrVparamss)
 
     val (normalizedBody, enumCases, enumCompanionRef) = {
@@ -475,7 +472,7 @@ object desugar {
             defDef(
               addEvidenceParams(
                 cpy.DefDef(ddef)(tparams = constrTparams ++ ddef.tparams),
-                evidenceParams(constr1).map(toDefParam(_, keepAnnotations = false)))))
+                evidenceParams(constr1).map(toDefParam(_)))))
         case stat =>
           stat
       }
@@ -499,7 +496,7 @@ object desugar {
 
     def anyRef = ref(defn.AnyRefAlias.typeRef)
 
-    // Annotations are dropped from the constructor parameters but should be
+    // Annotations are dropped from the constructor type parameters but should be
     // preserved in all derived parameters.
     val derivedTparams = {
       val impliedTparamsIt = impliedTparams.iterator