More efficient garbage collection of type variables

compiler/src/dotty/tools/dotc/core/Constraint.scala

@@ -135,8 +135,13 @@ abstract class Constraint extends Showable {
    */
   def uninstVars: collection.Seq[TypeVar]
 
-  /** The weakest constraint that subsumes both this constraint and `other` */
-  def & (other: Constraint)(implicit ctx: Context): Constraint
+  /** The weakest constraint that subsumes both this constraint and `other`.
+   *
+   *  @param otherHasErrors    If true, handle incompatible constraints by
+   *                           returning an approximate constraint, instead of
+   *                           failing with an exception
+   */
+  def & (other: Constraint, otherHasErrors: Boolean)(implicit ctx: Context): Constraint
 
   /** Check that no constrained parameter contains itself as a bound */
   def checkNonCyclic()(implicit ctx: Context): Unit

compiler/src/dotty/tools/dotc/core/ConstraintHandling.scala

@@ -63,29 +63,38 @@ trait ConstraintHandling {
       }
       if (Config.checkConstraintsSeparated)
         assert(!occursIn(bound), s"$param occurs in $bound")
-      val newBound = narrowedBound(param, bound, isUpper)
-      val c1 = constraint.updateEntry(param, newBound)
-      (c1 eq constraint) || {
-        constraint = c1
-        val TypeBounds(lo, hi) = constraint.entry(param)
-        isSubType(lo, hi)
+
+      val oldBounds @ TypeBounds(lo, hi) = constraint.nonParamBounds(param)
+      val equalBounds = isUpper && (lo eq bound) || !isUpper && (bound eq hi)
+      if (equalBounds && !bound.existsPart(_.isInstanceOf[WildcardType])) {
+        // The narrowed bounds are equal and do not contain wildcards,
+        // so we can remove `param` from the constraint.
+        // (Handling wildcards requires choosing a bound, but we don't know which
+        // bound to choose here, this is handled in `ConstraintHandling#approximation`)
+        constraint = constraint.replace(param, bound)
+        true
+      }
+      else {
+        // Narrow one of the bounds of type parameter `param`
+        // If `isUpper` is true, ensure that `param <: `bound`, otherwise ensure
+        // that `param >: bound`.
+        val narrowedBounds = {
+          val saved = homogenizeArgs
+          homogenizeArgs = Config.alignArgsInAnd
+          try
+            if (isUpper) oldBounds.derivedTypeBounds(lo, hi & bound)
+            else oldBounds.derivedTypeBounds(lo | bound, hi)
+          finally homogenizeArgs = saved
+        }
+        val c1 = constraint.updateEntry(param, narrowedBounds)
+        (c1 eq constraint) || {
+          constraint = c1
+          val TypeBounds(lo, hi) = constraint.entry(param)
+          isSubType(lo, hi)
+        }
       }
     }
 
-  /** Narrow one of the bounds of type parameter `param`
-   *  If `isUpper` is true, ensure that `param <: `bound`, otherwise ensure
-   *  that `param >: bound`.
-   */
-  def narrowedBound(param: TypeParamRef, bound: Type, isUpper: Boolean)(implicit ctx: Context): TypeBounds = {
-    val oldBounds @ TypeBounds(lo, hi) = constraint.nonParamBounds(param)
-    val saved = homogenizeArgs
-    homogenizeArgs = Config.alignArgsInAnd
-    try
-      if (isUpper) oldBounds.derivedTypeBounds(lo, hi & bound)
-      else oldBounds.derivedTypeBounds(lo | bound, hi)
-    finally homogenizeArgs = saved
-  }
-
   private def location(implicit ctx: Context) = "" // i"in ${ctx.typerState.stateChainStr}" // use for debugging
 
   protected def addUpperBound(param: TypeParamRef, bound: Type): Boolean = {

compiler/src/dotty/tools/dotc/core/OrderingConstraint.scala

@@ -501,7 +501,7 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
       }
     }
 
-  def & (other: Constraint)(implicit ctx: Context) = {
+  def & (other: Constraint, otherHasErrors: Boolean)(implicit ctx: Context) = {
     def merge[T](m1: ArrayValuedMap[T], m2: ArrayValuedMap[T], join: (T, T) => T): ArrayValuedMap[T] = {
       var merged = m1
       def mergeArrays(xs1: Array[T], xs2: Array[T]) = {
@@ -528,7 +528,10 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
         case (_, e2: TypeBounds) if e2 contains e1 => e1
         case (tv1: TypeVar, tv2: TypeVar) if tv1.instanceOpt eq tv2.instanceOpt => e1
         case _ =>
-          throw new AssertionError(i"cannot merge $this with $other, mergeEntries($e1, $e2) failed")
+          if (otherHasErrors)
+            e1
+          else
+            throw new AssertionError(i"cannot merge $this with $other, mergeEntries($e1, $e2) failed")
       }
 
     val that = other.asInstanceOf[OrderingConstraint]

compiler/src/dotty/tools/dotc/core/TyperState.scala

@@ -157,7 +157,7 @@ class TyperState(previous: TyperState /* | Null */) {
     assert(isCommittable)
     targetState.constraint =
       if (targetState.constraint eq previousConstraint) constraint
-      else targetState.constraint & constraint
+      else targetState.constraint & (constraint, otherHasErrors = reporter.errorsReported)
     constraint foreachTypeVar { tvar =>
       if (tvar.owningState.get eq this) tvar.owningState = new WeakReference(targetState)
     }

tests/pos-deep-subtype/inductive-implicits-bench.scala

@@ -0,0 +1,127 @@
+// Adapted from https://github.com/scala/compiler-benchmark/blob/master/corpus/induction/latest/inductive-implicits-bench.scala
+
+// With polymorphic implicit pruning:
+//   set resolvers in compilation += "pr-scala snapshots" at "https://scala-ci.typesafe.com/artifactory/scala-pr-validation-snapshots/"
+//   set scalaVersion in compilation := "2.13.0-pre-765b3ed-SNAPSHOT"
+//
+// Without polymorphic implicit pruning:
+//   set resolvers in compilation += "scala-integration" at "https://scala-ci.typesafe.com/artifactory/scala-integration/"
+//   set scalaVersion in compilation := "2.13.0-pre-1c56f0a"
+//
+// Then:
+//   cold -psource=induction -jvmArgs -Xss4M -jvmArgs -Xmx2G
+//
+// Nb. this is *very* slow without the pruning (> 400s).
+// With the pruning: 10-20s on reasonable hardware.
+
+package shapeless {
+  sealed trait HList extends Product with Serializable
+
+  final case class ::[+H, +T <: HList](head : H, tail : T) extends HList {
+    def ::[HH](h : HH) : HH :: H :: T = shapeless.::(h, this)
+
+    override def toString = head match {
+      case _: ::[_, _] => "("+head.toString+") :: "+tail.toString
+      case _ => head.toString+" :: "+tail.toString
+    }
+  }
+
+  sealed trait HNil extends HList {
+    def ::[H](h : H) = shapeless.::(h, this)
+    override def toString = "HNil"
+  }
+
+  case object HNil extends HNil
+
+  //@annotation.inductive
+  trait Selector[L <: HList, U] {
+    def apply(l: L): U
+  }
+
+  object Selector {
+    def apply[L <: HList, U](implicit selector: Selector[L, U]): Selector[L, U] = selector
+
+    implicit def inHead[H, T <: HList]: Selector[H :: T, H] =
+      new Selector[H :: T, H] {
+        def apply(l : H :: T) = l.head
+      }
+
+    implicit def inTail[H, T <: HList, U]
+      (implicit st : Selector[T, U]): Selector[H :: T, U] =
+        new Selector[H :: T, U] {
+          def apply(l : H :: T) = st(l.tail)
+        }
+  }
+}
+
+import shapeless._
+
+object Test extends App {
+  val sel = Selector[L, Boolean]
+
+  type L =
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    //
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    //
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    /*
+    //
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    //
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    //
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    //
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    //
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    //
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    //
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int :: Int ::
+    //
+    */
+    Boolean ::
+    HNil
+}