Add some logging to sinful typevar methods.

These super-mutation-oriented methods should enthusiastically
communicate what they are doing, especially when they encounter
anything unexpected. None of this work should be taken as an
endorsement of any of the worked-upon code.

3 files changed, 58 insertions, 48 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Contexts.scala b/src/compiler/scala/tools/nsc/typechecker/Contexts.scala
index eb91251930..26e39d3d1b 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Contexts.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Contexts.scala
@@ -8,6 +8,7 @@ package typechecker
 
 import scala.collection.mutable
 import scala.annotation.tailrec
+import scala.reflect.internal.util.shortClassOfInstance
 
 /**
  *  @author  Martin Odersky
@@ -175,6 +176,7 @@ trait Contexts { self: Analyzer =>
       if ((owner eq NoSymbol) || (owner.isClass) || (owner.isMethod)) this
       else outer.enclClassOrMethod
 
+    def enclosingCaseDef = nextEnclosing(_.tree.isInstanceOf[CaseDef])
     def undetparamsString =
       if (undetparams.isEmpty) ""
       else undetparams.mkString("undetparams=", ", ", "")
@@ -584,23 +586,39 @@ trait Contexts { self: Analyzer =>
     }
 
     def pushTypeBounds(sym: Symbol) {
+      sym.info match {
+        case tb: TypeBounds => if (!tb.isEmptyBounds) log(s"Saving $sym info=$tb")
+        case info           => devWarning(s"Something other than a TypeBounds seen in pushTypeBounds: $info is a ${shortClassOfInstance(info)}")
+      }
       savedTypeBounds ::= ((sym, sym.info))
     }
 
     def restoreTypeBounds(tp: Type): Type = {
-      var current = tp
-      for ((sym, info) <- savedTypeBounds) {
-        debuglog("resetting " + sym + " to " + info)
-        sym.info match {
-          case TypeBounds(lo, hi) if (hi <:< lo && lo <:< hi) =>
-            current = current.instantiateTypeParams(List(sym), List(lo))
-//@M TODO: when higher-kinded types are inferred, probably need a case PolyType(_, TypeBounds(...)) if ... =>
-          case _ =>
-        }
-        sym.setInfo(info)
+      def restore(): Type = savedTypeBounds.foldLeft(tp) { case (current, (sym, savedInfo)) =>
+        def bounds_s(tb: TypeBounds) = if (tb.isEmptyBounds) "<empty bounds>" else s"TypeBounds(lo=${tb.lo}, hi=${tb.hi})"
+        //@M TODO: when higher-kinded types are inferred, probably need a case PolyType(_, TypeBounds(...)) if ... =>
+        val tb @ TypeBounds(lo, hi) = sym.info.bounds
+        val isUnique                = lo <:< hi && hi <:< lo
+        val isPresent               = current contains sym
+        def saved_s                 = bounds_s(savedInfo.bounds)
+        def current_s               = bounds_s(sym.info.bounds)
+
+        if (isUnique && isPresent)
+          devWarningResult(s"Preserving inference: ${sym.nameString}=$hi in $current (based on $current_s) before restoring $sym to saved $saved_s")(
+            current.instantiateTypeParams(List(sym), List(hi))
+          )
+        else if (isPresent)
+          devWarningResult(s"Discarding inferred $current_s because it does not uniquely determine $sym in")(current)
+        else
+          logResult(s"Discarding inferred $current_s because $sym does not appear in")(current)
+      }
+      try restore()
+      finally {
+        for ((sym, savedInfo) <- savedTypeBounds)
+          sym setInfo debuglogResult(s"Discarding inferred $sym=${sym.info}, restoring saved info")(savedInfo)
+
+        savedTypeBounds = Nil
       }
-      savedTypeBounds = List()
-      current
     }
 
     private var implicitsCache: List[List[ImplicitInfo]] = null
diff --git a/src/compiler/scala/tools/nsc/typechecker/Infer.scala b/src/compiler/scala/tools/nsc/typechecker/Infer.scala
index db3759d65f..a29cc93b6d 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Infer.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Infer.scala
@@ -1317,15 +1317,18 @@ trait Infer extends Checkable {
       }
     }
 
-    def instBounds(tvar: TypeVar): (Type, Type) = {
-      val tparam = tvar.origin.typeSymbol
-      val instType = toOrigin(tvar.constr.inst)
+    def instBounds(tvar: TypeVar): TypeBounds = {
+      val tparam               = tvar.origin.typeSymbol
+      val instType             = toOrigin(tvar.constr.inst)
+      val TypeBounds(lo, hi)   = tparam.info.bounds
       val (loBounds, hiBounds) =
-        if (instType != NoType && isFullyDefined(instType)) (List(instType), List(instType))
+        if (isFullyDefined(instType)) (List(instType), List(instType))
         else (tvar.constr.loBounds, tvar.constr.hiBounds)
-      val lo = lub(tparam.info.bounds.lo :: loBounds map toOrigin)
-      val hi = glb(tparam.info.bounds.hi :: hiBounds map toOrigin)
-      (lo, hi)
+
+      TypeBounds(
+        lub(lo :: loBounds map toOrigin),
+        glb(hi :: hiBounds map toOrigin)
+      )
     }
 
     def isInstantiatable(tvars: List[TypeVar]) = {
@@ -1335,33 +1338,25 @@ trait Infer extends Checkable {
       solve(tvars1, tvars1 map (_.origin.typeSymbol), tvars1 map (_ => Variance.Covariant), false)
     }
 
-    // this is quite nasty: it destructively changes the info of the syms of e.g., method type params (see #3692, where the type param T's bounds were set to >: T <: T, so that parts looped)
+    // this is quite nasty: it destructively changes the info of the syms of e.g., method type params
+    // (see #3692, where the type param T's bounds were set to > : T <: T, so that parts looped)
     // the changes are rolled back by restoreTypeBounds, but might be unintentially observed in the mean time
     def instantiateTypeVar(tvar: TypeVar) {
-      val tparam = tvar.origin.typeSymbol
-      if (false &&
-          tvar.constr.inst != NoType &&
-          isFullyDefined(tvar.constr.inst) &&
-          (tparam.info.bounds containsType tvar.constr.inst)) {
-        context.nextEnclosing(_.tree.isInstanceOf[CaseDef]).pushTypeBounds(tparam)
-        tparam setInfo tvar.constr.inst
-        tparam resetFlag DEFERRED
-        debuglog("new alias of " + tparam + " = " + tparam.info)
-      } else {
-        val (lo, hi) = instBounds(tvar)
-        if (lo <:< hi) {
-          if (!((lo <:< tparam.info.bounds.lo) && (tparam.info.bounds.hi <:< hi)) // bounds were improved
-             && tparam != lo.typeSymbolDirect && tparam != hi.typeSymbolDirect) { // don't create illegal cycles
-            context.nextEnclosing(_.tree.isInstanceOf[CaseDef]).pushTypeBounds(tparam)
-            tparam setInfo TypeBounds(lo, hi)
-            debuglog("new bounds of " + tparam + " = " + tparam.info)
-          } else {
-            debuglog("redundant: "+tparam+" "+tparam.info+"/"+lo+" "+hi)
-          }
-        } else {
-          debuglog("inconsistent: "+tparam+" "+lo+" "+hi)
+      val tparam                    = tvar.origin.typeSymbol
+      val TypeBounds(lo0, hi0)      = tparam.info.bounds
+      val tb @ TypeBounds(lo1, hi1) = instBounds(tvar)
+
+      if (lo1 <:< hi1) {
+        if (lo1 <:< lo0 && hi0 <:< hi1) // bounds unimproved
+          log(s"redundant bounds: discarding TypeBounds($lo1, $hi1) for $tparam, no improvement on TypeBounds($lo0, $hi0)")
+        else if (tparam == lo1.typeSymbolDirect || tparam == hi1.typeSymbolDirect)
+          log(s"cyclical bounds: discarding TypeBounds($lo1, $hi1) for $tparam because $tparam appears as bounds")
+        else {
+          context.enclosingCaseDef pushTypeBounds tparam
+          tparam setInfo logResult(s"updated bounds: $tparam from ${tparam.info} to")(tb)
         }
       }
+      else log(s"inconsistent bounds: discarding TypeBounds($lo1, $hi1)")
     }
 
     /** Type intersection of simple type tp1 with general type tp2.
@@ -1524,7 +1519,7 @@ trait Infer extends Checkable {
         // todo: missing test case for bests.isEmpty
         bests match {
           case best :: Nil                              => tree setSymbol best setType (pre memberType best)
-          case best :: competing :: _ if alts0.nonEmpty => 
+          case best :: competing :: _ if alts0.nonEmpty =>
             // SI-6912 Don't give up and leave an OverloadedType on the tree.
             //         Originally I wrote this as `if (secondTry) ... `, but `tryTwice` won't attempt the second try
             //         unless an error is issued. We're not issuing an error, in the assumption that it would be
diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index c40b69bc7a..9680b911e0 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -2438,16 +2438,13 @@ trait Typers extends Adaptations with Tags {
                          else typed(cdef.guard, BooleanClass.tpe)
       var body1: Tree = typed(cdef.body, pt)
 
-      val contextWithTypeBounds = context.nextEnclosing(_.tree.isInstanceOf[CaseDef])
-      if (contextWithTypeBounds.savedTypeBounds.nonEmpty) {
-        body1 modifyType (contextWithTypeBounds restoreTypeBounds _)
-
+      if (context.enclosingCaseDef.savedTypeBounds.nonEmpty) {
+        body1 modifyType context.enclosingCaseDef.restoreTypeBounds
         // insert a cast if something typechecked under the GADT constraints,
         // but not in real life (i.e., now that's we've reset the method's type skolems'
         //   infos back to their pre-GADT-constraint state)
         if (isFullyDefined(pt) && !(body1.tpe <:< pt))
           body1 = typedPos(body1.pos)(gen.mkCast(body1, pt.dealiasWiden))
-
       }
 
 //    body1 = checkNoEscaping.locals(context.scope, pt, body1)