Fix #8531: Annnotations on class value parameters go to the constructor

Mimic the behavior of Scala 2: an annotation on a class value
parameters (but not on a type parameter) is moved to the constructor
parameter and not to any of the derived parameters.

Author: smarter

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

@@ -452,6 +452,10 @@ object desugar {
         else originalTparams
       }
       else originalTparams
+
+    // Annotations on class _type_ parameters are set on the derived parameters
+    // but not on the constructor parameters. The reverse is true for
+    // annotations on class _value_ parameters.
     val constrTparams = impliedTparams.map(toDefParam(_, keepAnnotations = false))
     val constrVparamss =
       if (originalVparamss.isEmpty) { // ensure parameter list is non-empty
@@ -463,7 +467,18 @@ object desugar {
         ctx.error(CaseClassMissingNonImplicitParamList(cdef), namePos)
         ListOfNil
       }
-      else originalVparamss.nestedMap(toDefParam(_, keepAnnotations = false))
+      else originalVparamss.nestedMap(toDefParam(_, keepAnnotations = true))
+    val derivedTparams = {
+      val impliedTparamsIt = impliedTparams.iterator
+      constrTparams.map(tparam => derivedTypeParam(tparam)
+        .withAnnotations(impliedTparamsIt.next().mods.annotations))
+    }
+    val derivedVparamss = {
+      val constrVparamsIt = constrVparamss.iterator.flatten
+      constrVparamss.nestedMap(vparam => derivedTermParam(vparam)
+        .withAnnotations(Nil))
+    }
+
     val constr = cpy.DefDef(constr1)(tparams = constrTparams, vparamss = constrVparamss)
 
     val (normalizedBody, enumCases, enumCompanionRef) = {
@@ -499,19 +514,6 @@ object desugar {
 
     def anyRef = ref(defn.AnyRefAlias.typeRef)
 
-    // Annotations are dropped from the constructor parameters but should be
-    // preserved in all derived parameters.
-    val derivedTparams = {
-      val impliedTparamsIt = impliedTparams.iterator
-      constrTparams.map(tparam => derivedTypeParam(tparam)
-        .withAnnotations(impliedTparamsIt.next().mods.annotations))
-    }
-    val derivedVparamss = {
-      val constrVparamsIt = constrVparamss.iterator.flatten
-      constrVparamss.nestedMap(vparam => derivedTermParam(vparam)
-        .withAnnotations(constrVparamsIt.next().mods.annotations))
-    }
-
     val arity = constrVparamss.head.length
 
     val classTycon: Tree = TypeRefTree() // watching is set at end of method
@@ -800,8 +802,10 @@ object desugar {
         val originalVparamsIt = originalVparamss.iterator.flatten
         derivedVparamss match {
           case first :: rest =>
-            first.map(_.withMods(originalVparamsIt.next().mods | caseAccessor)) ++
-            rest.flatten.map(_.withMods(originalVparamsIt.next().mods))
+            // Annotations on the class _value_ parameters are not set on the parameter accessors
+            def mods(vdef: ValDef) = vdef.mods.withAnnotations(Nil)
+            first.map(_.withMods(mods(originalVparamsIt.next()) | caseAccessor)) ++
+            rest.flatten.map(_.withMods(mods(originalVparamsIt.next())))
           case _ =>
             Nil
         }

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

@@ -228,12 +228,12 @@ class Constructors extends MiniPhase with IdentityDenotTransformer { thisPhase =
         Nil
       }
       else {
-        if (acc.hasAnnotation(defn.TransientParamAnnot))
+        val param = acc.subst(accessors, paramSyms)
+        if (param.hasAnnotation(defn.TransientParamAnnot))
           ctx.error(em"transient parameter $acc is retained as field in class ${acc.owner}", acc.sourcePos)
         val target = if (acc.is(Method)) acc.field else acc
         if (!target.exists) Nil // this case arises when the parameter accessor is an alias
         else {
-          val param = acc.subst(accessors, paramSyms)
           val assigns = Assign(ref(target), ref(param)).withSpan(tree.span) :: Nil
           if (acc.name != nme.OUTER) assigns
           else {