Value class Depth.

It's the obvious translation from a raw Int into a value class.

It wasn't that long ago one could find a signature like this:

  def merge(tps: List[Type], variance: Int, depth: Int): Type

Do you feel lucky, method caller? Well, do ya?

Anyway, now it is:

  def merge(tps: List[Type], variance: Variance, depth: Depth): Type

Forget for a moment the fact that you'd probably rather not pass variance
for depth and depth for variance and look at the type signatures:

  (List[Type], Variance, Depth) => Type
  (List[Type], Int, Int) => Type

Author: paulp

src/compiler/scala/reflect/macros/compiler/Validators.scala

@@ -57,7 +57,7 @@ trait Validators {
       checkMacroImplResultTypeMismatch(atpeToRtpe(aret), rret)
 
       val maxLubDepth = lubDepth(aparamss.flatten map (_.tpe)) max lubDepth(rparamss.flatten map (_.tpe))
-      val atargs = solvedTypes(atvars, atparams, atparams map varianceInType(aret), upper = false, depth = maxLubDepth)
+      val atargs = solvedTypes(atvars, atparams, atparams map varianceInType(aret), upper = false, maxLubDepth)
       val boundsOk = typer.silent(_.infer.checkBounds(macroDdef, NoPrefix, NoSymbol, atparams, atargs, ""))
       boundsOk match {
         case SilentResultValue(true) => // do nothing, success

src/compiler/scala/tools/nsc/typechecker/Implicits.scala

@@ -644,8 +644,7 @@ trait Implicits {
             if (tvars.nonEmpty)
               typingLog("solve", ptLine("tvars" -> tvars, "tvars.constr" -> tvars.map(_.constr)))
 
-            val targs = solvedTypes(tvars, undetParams, undetParams map varianceInType(pt),
-                                    upper = false, lubDepth(List(itree2.tpe, pt)))
+            val targs = solvedTypes(tvars, undetParams, undetParams map varianceInType(pt), upper = false, lubDepth(itree2.tpe :: pt :: Nil))
 
             // #2421: check that we correctly instantiated type parameters outside of the implicit tree:
             checkBounds(itree2, NoPrefix, NoSymbol, undetParams, targs, "inferred ")

src/compiler/scala/tools/nsc/typechecker/Infer.scala

@@ -9,6 +9,7 @@ package typechecker
 import scala.collection.{ mutable, immutable }
 import scala.util.control.ControlThrowable
 import symtab.Flags._
+import scala.reflect.internal.Depth
 
 /** This trait contains methods related to type parameter inference.
  *
@@ -21,6 +22,7 @@ trait Infer extends Checkable {
   import global._
   import definitions._
   import typeDebug.ptBlock
+  import typeDebug.str.parentheses
   import typingStack.{ printTyping }
 
   /** The formal parameter types corresponding to `formals`.
@@ -132,34 +134,17 @@ trait Infer extends Checkable {
    *  @param upper      When `true` search for max solution else min.
    *  @throws NoInstance
    */
-  def solvedTypes(tvars: List[TypeVar], tparams: List[Symbol],
-                  variances: List[Variance], upper: Boolean, depth: Int): List[Type] = {
-
-    if (tvars.nonEmpty) {
-      def tp_s = (tparams, tvars).zipped map { case (tp, tv) => s"${tp.name}/$tv" } mkString ","
-      printTyping(s"solving for $tp_s")
-    }
-
-    if (!solve(tvars, tparams, variances, upper, depth)) {
-      // no panic, it's good enough to just guess a solution, we'll find out
-      // later whether it works.  *ZAP* @M danger, Will Robinson! this means
-      // that you should never trust inferred type arguments!
-      //
-      // Need to call checkBounds on the args/typars or type1 on the tree
-      // for the expression that results from type inference see e.g., #2421:
-      // implicit search had been ignoring this caveat
-      // throw new DeferredNoInstance(() =>
-      //   "no solution exists for constraints"+(tvars map boundsString))
+  def solvedTypes(tvars: List[TypeVar], tparams: List[Symbol], variances: List[Variance], upper: Boolean, depth: Depth): List[Type] = {
+    if (tvars.isEmpty) Nil else {
+      printTyping("solving for " + parentheses((tparams, tvars).zipped map ((p, tv) => s"${p.name}: $tv")))
+      // !!! What should be done with the return value of "solve", which is at present ignored?
+      // The historical commentary says "no panic, it's good enough to just guess a solution,
+      // we'll find out later whether it works", meaning don't issue an error here when types
+      // don't conform to bounds. That means you can never trust the results of implicit search.
+      // For an example where this was not being heeded, SI-2421.
+      solve(tvars, tparams, variances, upper, depth)
+      tvars map instantiate
     }
-    for (tvar <- tvars ; if tvar.constr.inst == tvar) {
-      if (tvar.origin.typeSymbol.info eq ErrorType)
-        // this can happen if during solving a cyclic type parameter
-        // such as T <: T gets completed. See #360
-        tvar.constr.inst = ErrorType
-      else
-        abort(tvar.origin+" at "+tvar.origin.typeSymbol.owner)
-    }
-    tvars map instantiate
   }
 
   def skipImplicit(tp: Type) = tp match {
@@ -554,10 +539,7 @@ trait Infer extends Checkable {
             "argument expression's type is not compatible with formal parameter type" + foundReqMsg(tp1, pt1))
         }
       }
-      val targs = solvedTypes(
-        tvars, tparams, tparams map varianceInTypes(formals),
-        upper = false, lubDepth(formals) max lubDepth(argtpes)
-      )
+      val targs = solvedTypes(tvars, tparams, tparams map varianceInTypes(formals), upper = false, lubDepth(formals) max lubDepth(argtpes))
       // Can warn about inferring Any/AnyVal as long as they don't appear
       // explicitly anywhere amongst the formal, argument, result, or expected type.
       def canWarnAboutAny = !(pt :: restpe :: formals ::: argtpes exists (t => (t contains AnyClass) || (t contains AnyValClass)))
@@ -1030,7 +1012,10 @@ trait Infer extends Checkable {
             val variances  =
               if (ctorTp.paramTypes.isEmpty) undetparams map varianceInType(ctorTp)
               else undetparams map varianceInTypes(ctorTp.paramTypes)
-            val targs      = solvedTypes(tvars, undetparams, variances, upper = true, lubDepth(List(resTp, pt)))
+
+            // Note: this is the only place where solvedTypes (or, indirectly, solve) is called
+            // with upper = true.
+            val targs = solvedTypes(tvars, undetparams, variances, upper = true, lubDepth(resTp :: pt :: Nil))
             // checkBounds(tree, NoPrefix, NoSymbol, undetparams, targs, "inferred ")
             // no checkBounds here. If we enable it, test bug602 fails.
             // TODO: reinstate checkBounds, return params that fail to meet their bounds to undetparams
@@ -1099,7 +1084,7 @@ trait Infer extends Checkable {
       val tvars1 = tvars map (_.cloneInternal)
       // Note: right now it's not clear that solving is complete, or how it can be made complete!
       // So we should come back to this and investigate.
-      solve(tvars1, tvars1 map (_.origin.typeSymbol), tvars1 map (_ => Variance.Covariant), upper = false)
+      solve(tvars1, tvars1 map (_.origin.typeSymbol), tvars1 map (_ => Variance.Covariant), upper = false, Depth.AnyDepth)
     }
 
     // this is quite nasty: it destructively changes the info of the syms of e.g., method type params

src/reflect/scala/reflect/internal/BaseTypeSeqs.scala

@@ -130,9 +130,9 @@ trait BaseTypeSeqs {
 
     lazy val maxDepth = maxDepthOfElems
 
-    protected def maxDepthOfElems: Int = {
-      var d = 0
-      for (i <- 1 until length) d = max(d, typeDepth(elems(i)))
+    protected def maxDepthOfElems: Depth = {
+      var d = Depth.Zero
+      1 until length foreach (i => d = d max typeDepth(elems(i)))
       d
     }
 
@@ -234,7 +234,7 @@ trait BaseTypeSeqs {
     override def map(g: Type => Type) = lateMap(g)
     override def lateMap(g: Type => Type) = orig.lateMap(x => g(f(x)))
     override def exists(p: Type => Boolean) = elems exists (x => p(f(x)))
-    override protected def maxDepthOfElems: Int = elems.map(x => typeDepth(f(x))).max
+    override protected def maxDepthOfElems: Depth = elems.map(x => typeDepth(f(x))).max
     override def toString = elems.mkString("MBTS(", ",", ")")
   }
 

src/reflect/scala/reflect/internal/Depth.scala

@@ -0,0 +1,28 @@
+package scala
+package reflect
+package internal
+
+import Depth._
+
+final class Depth private (val depth: Int) extends AnyVal with Ordered[Depth] {
+  def max(that: Depth): Depth   = if (this < that) that else this
+  def decr(n: Int): Depth       = if (isAnyDepth) this else Depth(depth - n)
+  def incr(n: Int): Depth       = if (isAnyDepth) this else Depth(depth + n)
+  def decr: Depth               = decr(1)
+  def incr: Depth               = incr(1)
+
+  def isNegative = depth < 0
+  def isZero     = depth == 0
+  def isAnyDepth = this == AnyDepth
+
+  def compare(that: Depth): Int = if (depth < that.depth) -1 else if (this == that) 0 else 1
+  override def toString = s"Depth($depth)"
+}
+
+object Depth {
+  // A don't care value for the depth parameter in lubs/glbs and related operations.
+  final val AnyDepth = new Depth(Int.MinValue)
+  final val Zero     = new Depth(0)
+
+  @inline final def apply(depth: Int): Depth = if (depth < 0) AnyDepth else new Depth(depth)
+}