Fix trait encoding for private vars in traits.

Author: sjrd

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

@@ -9,6 +9,7 @@ import dotty.tools.dotc.core.StdNames._
 import ast._
 import Trees._
 import Flags._
+import NameOps._
 import SymUtils._
 import Symbols._
 import Decorators._
@@ -211,8 +212,22 @@ class Constructors extends MiniPhase with IdentityDenotTransformer { thisPhase =
             val sym = stat.symbol
             assert(isRetained(sym), sym)
             if (!stat.rhs.isEmpty && !isWildcardArg(stat.rhs))
+              /* !!! This should really just be `sym.setter`. However, if we do that, we'll miss
+               * setters for mixed in `private var`s. Even though the scope clearly contains the
+               * setter symbol with the correct Name structure (since the `find` finds it),
+               * `.decl(setterName)` used by `.setter` through `.accessorNamed` will *not* find it.
+               * Could it be that the hash table of the `Scope` is corrupted?
+               * We still try `sym.setter` first as an optimization, since it will work for all
+               * public vars in traits and all (public or private) vars in classes.
+               */
+              val symSetter =
+                if sym.setter.exists then
+                  sym.setter
+                else
+                  val setterName = sym.asTerm.name.setterName
+                  sym.owner.info.decls.find(d => d.is(Accessor) && d.name == setterName)
               val setter =
-                if (sym.setter.exists) sym.setter
+                if (symSetter.exists) symSetter
                 else sym.accessorNamed(Mixin.traitSetterName(sym.asTerm))
               constrStats += Apply(ref(setter), intoConstr(stat.rhs, sym).withSpan(stat.span) :: Nil)
             clsStats += cpy.DefDef(stat)(rhs = EmptyTree)

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

@@ -175,7 +175,8 @@ class Mixin extends MiniPhase with SymTransformer { thisPhase =>
     atPhase(thisPhase) { sym.isOneOf(flags) }
 
   private def needsTraitSetter(sym: Symbol)(using Context): Boolean =
-    sym.isGetter && !wasOneOf(sym, DeferredOrLazy | ParamAccessor) && !sym.setter.exists
+    sym.isGetter && !wasOneOf(sym, DeferredOrLazy | ParamAccessor)
+      && atPhase(thisPhase) { !sym.setter.exists }
       && !sym.info.resultType.isInstanceOf[ConstantType]
 
   private def makeTraitSetter(getter: TermSymbol)(using Context): Symbol =