Mixin: generate a getter in implementing class only if current

A class might implement several fields in inherited traits with the same and type.
In that case only one getter should be produced, but all initializing expressions
have to be executed.

Author: odersky

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

@@ -50,15 +50,15 @@ import Decorators._
 
   override def transformDefDef(tree: DefDef)(implicit ctx: Context, info: TransformerInfo): Tree = {
     val sym = tree.symbol
-    
+
     def newField = ctx.newSymbol(
       owner = ctx.owner,
       name = sym.name.asTermName.fieldName,
       flags = Private | (if (sym is Stable) EmptyFlags else Mutable),
       info = sym.info.resultType,
       coord = tree.pos).enteredAfter(thisTransform)
-      
-    lazy val field = sym.field.orElse(newField).asTerm    
+
+    lazy val field = sym.field.orElse(newField).asTerm
     if (sym.is(Accessor, butNot = NoFieldNeeded))
       if (sym.isGetter) {
         tree.rhs.changeOwnerAfter(sym, field, thisTransform)

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

@@ -153,7 +153,12 @@ class Mixin extends MiniPhaseTransform with SymTransformer { thisTransform =>
       for (getter <- mixin.info.decls.filter(getr => getr.isGetter && !wasDeferred(getr)).toList)
         yield {
           // transformFollowing call is needed to make memoize & lazy vals run
-        transformFollowing(DefDef(implementation(getter.asTerm), superRef(initializer(getter)).appliedToNone))
+        val rhs = transformFollowing(superRef(initializer(getter)).appliedToNone)
+        val isCurrent = ctx.atPhase(thisTransform) { implicit ctx =>
+          cls.info.member(getter.name).suchThat(_.isGetter).symbol == getter
+        }
+        if (isCurrent) transformFollowing(DefDef(implementation(getter.asTerm), rhs))
+        else rhs
       }
 
     def setters(mixin: ClassSymbol): List[Tree] =