incoherent cyclic references between synthesized members

must replace old trait accessor symbols by mixed in symbols
in the infos of the mixed in symbols

```
trait T { val a: String ; val b: a.type }
class C extends T {
  // a, b synthesized, but the a in b's type, a.type, refers to the original symbol, not the clone in C
}
```

symbols occurring in types of synthesized members
do not get rebound to other synthesized symbols

package <empty>#4 {
  abstract <defaultparam/trait> trait N#7352 extends scala#22.AnyRef#2378 {
    <method> <deferred> <mutable> <accessor> <triedcooking> <sub_synth> def N$_setter_$self_$eq#15011(x$1#15012: <empty>#3.this.N#7352): scala#23.this.Unit#2340;
    <method> <deferred> <mutable> <accessor> <triedcooking> <sub_synth> def N$_setter_$n_$eq#15013(x$1#15014: N#7352.this.self#7442.type): scala#23.this.Unit#2340;
    <method> def /*N*/$init$scala#7441(): scala#23.this.Unit#2340 = {
      ()
    };
    <method> <deferred> <stable> <accessor> <triedcooking> <sub_synth> def self#7442: <empty>#3.this.N#7352;
    N#7352.this.N$_setter_$self_$eq#15011(scala#22.Predef#1729.$qmark$qmark$qmark#6917);
    <method> <deferred> <stable> <accessor> <sub_synth> def n#7443: N#7352.this.self#7442.type;
    N#7352.this.N$_setter_$n_$eq#15013(N#7352.this.self#7442)
  };
  abstract class M#7353 extends scala#22.AnyRef#2378 {
    <method> <triedcooking> def <init>#13465(): <empty>#3.this.M#7353 = {
      M#7353.super.<init>scala#2719();
      ()
    };
    <method> <deferred> <stable> <accessor> <triedcooking> def self#13466: <empty>#3.this.N#7352;
    <method> <deferred> <stable> <accessor> def n#13467: M#7353.this.self#13466.type
  };
  class C#7354 extends M#7353 with <empty>#3.this.N#7352 {
    <method> <stable> <accessor> <triedcooking> def self#15016: <empty>#3.this.N#7352 = C#7354.this.self #15015;
    <triedcooking> private[this] val self #15015: <empty>#3.this.N#7352 = _;
    <method> <stable> <accessor> def n#15018: C#7354.this.self#7442.type = C#7354.this.n #15017;
    <triedcooking> private[this] val n #15017: C#7354.this.self#7442.type = _;
    <method> <mutable> <accessor> <triedcooking> def N$_setter_$self_$eq#15019(x$1#15021: <empty>#3.this.N#7352): scala#23.this.Unit#2340 = C#7354.this.self #15015 = x$1#15021;
    <method> <mutable> <accessor> <triedcooking> def N$_setter_$n_$eq#15022(x$1#15025: C#7354.this.self#7442.type): scala#23.this.Unit#2340 = C#7354.this.n #15017 = x$1#15025;
    <method> def <init>#14997(): <empty>#3.this.C#7354 = {
      C#7354.super.<init>#13465();
      ()
    }
  }
}

[running phase pickler on dependent_rebind.scala]
[running phase refchecks on dependent_rebind.scala]
test/files/trait-defaults/dependent_rebind.scala:16: error: overriding field n#15049 in trait N#7352 of type C#7354.this.self#15016.type;
 value n #15017 has incompatible type;
 found   : => C#7354.this.self#7442.type (with underlying type => C#7354.this.self#7442.type)
 required: => N#7352.this.self#7442.type
class C extends M with N
      ^

Author: adriaanm

src/compiler/scala/tools/nsc/transform/Fields.scala

@@ -210,19 +210,20 @@ abstract class Fields extends InfoTransform with ast.TreeDSL with TypingTransfor
         //  class OverridingVal extends OneVal[Int] { override val x: Int = ??? }
 
 
-        // mixin field accessors
-        val mixedInFieldAndAccessors = accessorsMaybeNeedingImpl flatMap { accessor =>
+        // mixin field accessors --
+        // invariant: (accessorsMaybeNeedingImpl, mixedInAccessorAndFields).zipped.forall(case (acc, clone :: _) => `clone` is clone of `acc` case _ => true)
+        val mixedInAccessorAndFields = accessorsMaybeNeedingImpl map { accessor =>
           def cloneAccessor() = {
             val clonedAccessor = (accessor cloneSymbol clazz) setPos clazz.pos setFlag NEEDS_TREES resetFlag DEFERRED | SYNTHESIZE_IMPL_IN_SUBCLASS | FINAL_TRAIT_ACCESSOR
             if (accessor hasFlag FINAL_TRAIT_ACCESSOR) {
               clonedAccessor setFlag FINAL | lateFINAL // lateFINAL thrown in for good measure, by analogy to makeNotPrivate
             }
             // if we don't cloneInfo, method argument symbols are shared between trait and subclasses --> lambalift proxy crash
             // TODO: use derive symbol variant?
-            val clonedInfo = accessor.info.cloneInfo(clonedAccessor)
-            val relativeInfo = clonedInfo.asSeenFrom(clazz.thisType, accessor.owner)
-//            println(s"cloning accessor $accessor to $clazz / $clonedInfo -> $relativeInfo")
-            clonedAccessor setInfo relativeInfo
+
+            //            println(s"cloning accessor $accessor to $clazz / $clonedInfo -> $relativeInfo")
+            clonedAccessor setInfo ((clazz.thisType memberType accessor) cloneInfo clonedAccessor) // accessor.info.cloneInfo(clonedAccessor).asSeenFrom(clazz.thisType, accessor.owner)
+
           }
 
           // when considering whether to mix in the trait setter, forget about conflicts -- they will be reported for the getter
@@ -247,6 +248,7 @@ abstract class Fields extends InfoTransform with ast.TreeDSL with TypingTransfor
           else if (accessorConflictsExistingVal(accessor) || isOverriddenAccessor(accessor, clazz)) Nil
           else if (accessor.isGetter && fieldMemoizationIn(accessor, clazz).needsField) {
             // add field if needed
+
             val field = clazz.newValue(accessor.localName, accessor.pos) setInfo fieldTypeForGetterIn(accessor, clazz.thisType)
 
             val newFlags = (
@@ -264,19 +266,28 @@ abstract class Fields extends InfoTransform with ast.TreeDSL with TypingTransfor
           } else List(cloneAccessor())
         }
 
-//        println(s"new decls for $clazz: $mixedInFieldAndAccessors")
+//        println(s"new decls for $clazz: $mixedInAccessorAndFields")
 
         // omit fields that are not memoized, retain all other members
         def omittableField(sym: Symbol) = sym.isValue && !sym.isMethod && fieldMemoizationIn(sym, clazz).effectOnly // TODO: not yet `needsField`, to produce same bytecode as M2
 
         val newDecls =
-          if (mixedInFieldAndAccessors.isEmpty) oldDecls.filterNot(omittableField)
+          if (mixedInAccessorAndFields.isEmpty) oldDecls.filterNot(omittableField)
           else {  // must not alter `decls` directly
             val newDecls = newScope
             oldDecls foreach { d => if (!omittableField(d)) newDecls.enter(d) }
-            mixedInFieldAndAccessors foreach newDecls.enter
+            val enter = { mixedin: Symbol => newDecls enter mixedin }
+            mixedInAccessorAndFields foreach { _ foreach enter }
+
+            // subst from accessors to corresponding clonedAccessors in types in newDecls
+            val (origs, mixedins) = (accessorsMaybeNeedingImpl, mixedInAccessorAndFields).zipped.collect {
+              case (traitAccessor, mixedin :: _) => (traitAccessor, mixedin)
+            }.unzip
+            newDecls foreach { sym => sym.substInfo(origs.toList, mixedins.toList) }
+
             newDecls
           }
+
 //        println(s"new decls: $newDecls")
 
         if (newDecls eq oldDecls) tp
@@ -294,13 +305,13 @@ abstract class Fields extends InfoTransform with ast.TreeDSL with TypingTransfor
     def mkTypedUnit(pos: Position) = localTyper.typedPos(pos)(CODE.UNIT)
     def deriveUnitDef(stat: Tree)  = deriveDefDef(stat)(_ => mkTypedUnit(stat.pos))
 
-    // synth trees for mixed in accessors/fields and trait setters
+    // synth trees for accessors/fields and trait setters when they are mixed into a class
     def fieldsAndAccessors(templateSym: Symbol): List[ValOrDefDef] = {
       val clazz = templateSym.owner
       def fieldAccess(accessor: Symbol) = {
         val fieldName = accessor.localName
         val field = clazz.info.decl(fieldName)
-        assert(field.exists, s"Field '$fieldName' not found in ${clazz.info.decls}")
+        assert(field.exists, s"Field '$fieldName' not found in ${clazz.info.decls} of $clazz for $accessor")
         Select(This(clazz), field)
       }
 

test/files/trait-defaults/dependent_rebind.scala

@@ -0,0 +1,17 @@
+// derived from test/files/pos/S5.scala
+
+// compile with -uniqid to see a hint of the trouble
+trait N {
+  // the symbol for self does not get rebound when synthesizing members in C
+  val self: N = ???
+  val n: self.type = self
+}
+
+// uncomment extends clause for another bug
+abstract class M /*extends N*/ {
+  val self: N
+  val n: self.type
+}
+
+class C extends M with N
+