Eliminate class hierarchy in GadtConstraint (#16194)

Author: dwijnand

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

@@ -814,7 +814,7 @@ object Contexts {
       .updated(notNullInfosLoc, Nil)
       .updated(compilationUnitLoc, NoCompilationUnit)
     searchHistory = new SearchRoot
-    gadt = EmptyGadtConstraint
+    gadt = GadtConstraint.empty
   }
 
   @sharable object NoContext extends Context((null: ContextBase | Null).uncheckedNN) {

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

@@ -10,77 +10,25 @@ import util.{SimpleIdentitySet, SimpleIdentityMap}
 import collection.mutable
 import printing._
 
-import scala.annotation.internal.sharable
+object GadtConstraint:
+  def apply(): GadtConstraint = empty
+  def empty: GadtConstraint =
+    new ProperGadtConstraint(OrderingConstraint.empty, SimpleIdentityMap.empty, SimpleIdentityMap.empty, false)
 
 /** Represents GADT constraints currently in scope */
-sealed abstract class GadtConstraint extends Showable {
-  /** Immediate bounds of `sym`. Does not contain lower/upper symbols (see [[fullBounds]]). */
-  def bounds(sym: Symbol)(using Context): TypeBounds | Null
-
-  /** Full bounds of `sym`, including TypeRefs to other lower/upper symbols.
-   *
-   * @note this performs subtype checks between ordered symbols.
-   *       Using this in isSubType can lead to infinite recursion. Consider `bounds` instead.
-   */
-  def fullBounds(sym: Symbol)(using Context): TypeBounds | Null
-
-  /** Is `sym1` ordered to be less than `sym2`? */
-  def isLess(sym1: Symbol, sym2: Symbol)(using Context): Boolean
-
-  /** Add symbols to constraint, correctly handling inter-dependencies.
-   *
-   * @see [[ConstraintHandling.addToConstraint]]
-   */
-  def addToConstraint(syms: List[Symbol])(using Context): Boolean
-  def addToConstraint(sym: Symbol)(using Context): Boolean = addToConstraint(sym :: Nil)
-
-  /** Further constrain a symbol already present in the constraint. */
-  def addBound(sym: Symbol, bound: Type, isUpper: Boolean)(using Context): Boolean
-
-  /** Is the symbol registered in the constraint?
-   *
-   * @note this is true even if the symbol is constrained to be equal to another type, unlike [[Constraint.contains]].
-   */
-  def contains(sym: Symbol)(using Context): Boolean
-
-  /** GADT constraint narrows bounds of at least one variable */
-  def isNarrowing: Boolean
-
-  /** See [[ConstraintHandling.approximation]] */
-  def approximation(sym: Symbol, fromBelow: Boolean, maxLevel: Int = Int.MaxValue)(using Context): Type
-
-  def symbols: List[Symbol]
-
-  def fresh: GadtConstraint
-
-  /** Restore the state from other [[GadtConstraint]], probably copied using [[fresh]] */
-  def restore(other: GadtConstraint): Unit
-
-  /** Provides more information than toText, by showing the underlying Constraint details. */
-  def debugBoundsDescription(using Context): String
-}
-
-final class ProperGadtConstraint private(
+sealed trait GadtConstraint (
   private var myConstraint: Constraint,
   private var mapping: SimpleIdentityMap[Symbol, TypeVar],
   private var reverseMapping: SimpleIdentityMap[TypeParamRef, Symbol],
   private var wasConstrained: Boolean
-) extends GadtConstraint with ConstraintHandling {
-  import dotty.tools.dotc.config.Printers.{gadts, gadtsConstr}
+) extends Showable {
+  this: ConstraintHandling =>
 
-  def this() = this(
-    myConstraint = new OrderingConstraint(SimpleIdentityMap.empty, SimpleIdentityMap.empty, SimpleIdentityMap.empty, SimpleIdentitySet.empty),
-    mapping = SimpleIdentityMap.empty,
-    reverseMapping = SimpleIdentityMap.empty,
-    wasConstrained = false
-  )
+  import dotty.tools.dotc.config.Printers.{gadts, gadtsConstr}
 
   /** Exposes ConstraintHandling.subsumes */
   def subsumes(left: GadtConstraint, right: GadtConstraint, pre: GadtConstraint)(using Context): Boolean = {
-    def extractConstraint(g: GadtConstraint) = g match {
-      case s: ProperGadtConstraint => s.constraint
-      case EmptyGadtConstraint => OrderingConstraint.empty
-    }
+    def extractConstraint(g: GadtConstraint) = g.constraint
     subsumes(extractConstraint(left), extractConstraint(right), extractConstraint(pre))
   }
 
@@ -89,7 +37,12 @@ final class ProperGadtConstraint private(
     // the case where they're valid, so no approximating is needed.
     rawBound
 
-  override def addToConstraint(params: List[Symbol])(using Context): Boolean = {
+  /** Add symbols to constraint, correctly handling inter-dependencies.
+   *
+   * @see [[ConstraintHandling.addToConstraint]]
+   */
+  def addToConstraint(sym: Symbol)(using Context): Boolean = addToConstraint(sym :: Nil)
+  def addToConstraint(params: List[Symbol])(using Context): Boolean = {
     import NameKinds.DepParamName
 
     val poly1 = PolyType(params.map { sym => DepParamName.fresh(sym.name.toTypeName) })(
@@ -138,7 +91,8 @@ final class ProperGadtConstraint private(
       .showing(i"added to constraint: [$poly1] $params%, % gadt = $this", gadts)
   }
 
-  override def addBound(sym: Symbol, bound: Type, isUpper: Boolean)(using Context): Boolean = {
+  /** Further constrain a symbol already present in the constraint. */
+  def addBound(sym: Symbol, bound: Type, isUpper: Boolean)(using Context): Boolean = {
     @annotation.tailrec def stripInternalTypeVar(tp: Type): Type = tp match {
       case tv: TypeVar =>
         val inst = constraint.instType(tv)
@@ -179,10 +133,16 @@ final class ProperGadtConstraint private(
     result
   }
 
-  override def isLess(sym1: Symbol, sym2: Symbol)(using Context): Boolean =
+  /** Is `sym1` ordered to be less than `sym2`? */
+  def isLess(sym1: Symbol, sym2: Symbol)(using Context): Boolean =
     constraint.isLess(tvarOrError(sym1).origin, tvarOrError(sym2).origin)
 
-  override def fullBounds(sym: Symbol)(using Context): TypeBounds | Null =
+  /** Full bounds of `sym`, including TypeRefs to other lower/upper symbols.
+   *
+   * @note this performs subtype checks between ordered symbols.
+   *       Using this in isSubType can lead to infinite recursion. Consider `bounds` instead.
+   */
+  def fullBounds(sym: Symbol)(using Context): TypeBounds | Null =
     mapping(sym) match {
       case null => null
       // TODO: Improve flow typing so that ascription becomes redundant
@@ -191,7 +151,8 @@ final class ProperGadtConstraint private(
           // .ensuring(containsNoInternalTypes(_))
     }
 
-  override def bounds(sym: Symbol)(using Context): TypeBounds | Null =
+  /** Immediate bounds of `sym`. Does not contain lower/upper symbols (see [[fullBounds]]). */
+  def bounds(sym: Symbol)(using Context): TypeBounds | Null =
     mapping(sym) match {
       case null => null
       // TODO: Improve flow typing so that ascription becomes redundant
@@ -202,11 +163,17 @@ final class ProperGadtConstraint private(
           //.ensuring(containsNoInternalTypes(_))
     }
 
-  override def contains(sym: Symbol)(using Context): Boolean = mapping(sym) != null
+  /** Is the symbol registered in the constraint?
+   *
+   * @note this is true even if the symbol is constrained to be equal to another type, unlike [[Constraint.contains]].
+   */
+  def contains(sym: Symbol)(using Context): Boolean = mapping(sym) != null
 
+  /** GADT constraint narrows bounds of at least one variable */
   def isNarrowing: Boolean = wasConstrained
 
-  override def approximation(sym: Symbol, fromBelow: Boolean, maxLevel: Int)(using Context): Type = {
+  /** See [[ConstraintHandling.approximation]] */
+  def approximation(sym: Symbol, fromBelow: Boolean, maxLevel: Int = Int.MaxValue)(using Context): Type = {
     val res =
       approximation(tvarOrError(sym).origin, fromBelow, maxLevel) match
         case tpr: TypeParamRef =>
@@ -220,23 +187,16 @@ final class ProperGadtConstraint private(
     res
   }
 
-  override def symbols: List[Symbol] = mapping.keys
-
-  override def fresh: GadtConstraint = new ProperGadtConstraint(
-    myConstraint,
-    mapping,
-    reverseMapping,
-    wasConstrained
-  )
-
-  def restore(other: GadtConstraint): Unit = other match {
-    case other: ProperGadtConstraint =>
-      this.myConstraint = other.myConstraint
-      this.mapping = other.mapping
-      this.reverseMapping = other.reverseMapping
-      this.wasConstrained = other.wasConstrained
-    case _ => ;
-  }
+  def symbols: List[Symbol] = mapping.keys
+
+  def fresh: GadtConstraint = new ProperGadtConstraint(myConstraint, mapping, reverseMapping, wasConstrained)
+
+  /** Restore the state from other [[GadtConstraint]], probably copied using [[fresh]] */
+  def restore(other: GadtConstraint): Unit =
+    this.myConstraint = other.myConstraint
+    this.mapping = other.mapping
+    this.reverseMapping = other.reverseMapping
+    this.wasConstrained = other.wasConstrained
 
   // ---- Protected/internal -----------------------------------------------
 
@@ -294,30 +254,13 @@ final class ProperGadtConstraint private(
 
   override def toText(printer: Printer): Texts.Text = printer.toText(this)
 
-  override def debugBoundsDescription(using Context): String = i"$this\n$constraint"
+  /** Provides more information than toText, by showing the underlying Constraint details. */
+  def debugBoundsDescription(using Context): String = i"$this\n$constraint"
 }
 
-@sharable object EmptyGadtConstraint extends GadtConstraint {
-  override def bounds(sym: Symbol)(using Context): TypeBounds | Null = null
-  override def fullBounds(sym: Symbol)(using Context): TypeBounds | Null = null
-
-  override def isLess(sym1: Symbol, sym2: Symbol)(using Context): Boolean = unsupported("EmptyGadtConstraint.isLess")
-
-  override def isNarrowing: Boolean = false
-
-  override def contains(sym: Symbol)(using Context) = false
-
-  override def addToConstraint(params: List[Symbol])(using Context): Boolean = unsupported("EmptyGadtConstraint.addToConstraint")
-  override def addBound(sym: Symbol, bound: Type, isUpper: Boolean)(using Context): Boolean = unsupported("EmptyGadtConstraint.addBound")
-
-  override def approximation(sym: Symbol, fromBelow: Boolean, maxLevel: Int)(using Context): Type = unsupported("EmptyGadtConstraint.approximation")
-
-  override def symbols: List[Symbol] = Nil
-
-  override def fresh = new ProperGadtConstraint
-  override def restore(other: GadtConstraint): Unit =
-    assert(!other.isNarrowing, "cannot restore a non-empty GADTMap")
-
-  override def toText(printer: Printer): Texts.Text = printer.toText(this)
-  override def debugBoundsDescription(using Context): String = i"$this"
-}
+private class ProperGadtConstraint (
+    myConstraint: Constraint,
+    mapping: SimpleIdentityMap[Symbol, TypeVar],
+    reverseMapping: SimpleIdentityMap[TypeParamRef, Symbol],
+    wasConstrained: Boolean,
+) extends ConstraintHandling with GadtConstraint(myConstraint, mapping, reverseMapping, wasConstrained)

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

@@ -1848,11 +1848,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
     val preGadt = ctx.gadt.fresh
 
     def allSubsumes(leftGadt: GadtConstraint, rightGadt: GadtConstraint, left: Constraint, right: Constraint): Boolean =
-      subsumes(left, right, preConstraint) && preGadt.match
-        case preGadt: ProperGadtConstraint =>
-          preGadt.subsumes(leftGadt, rightGadt, preGadt)
-        case _ =>
-          true
+      subsumes(left, right, preConstraint) && preGadt.subsumes(leftGadt, rightGadt, preGadt)
 
     if op1 then
       val op1Constraint = constraint

compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala

@@ -694,13 +694,11 @@ class PlainPrinter(_ctx: Context) extends Printer {
     finally
       ctx.typerState.constraint = savedConstraint
 
-  def toText(g: GadtConstraint): Text = g match
-    case EmptyGadtConstraint     => "EmptyGadtConstraint"
-    case g: ProperGadtConstraint =>
-      val deps = for sym <- g.symbols yield
-        val bound = g.fullBounds(sym).nn
-        (typeText(toText(sym.typeRef)) ~ toText(bound)).close
-      ("GadtConstraint(" ~ Text(deps, ", ") ~ ")").close
+  def toText(g: GadtConstraint): Text =
+    val deps = for sym <- g.symbols yield
+      val bound = g.fullBounds(sym).nn
+      (typeText(toText(sym.typeRef)) ~ toText(bound)).close
+    ("GadtConstraint(" ~ Text(deps, ", ") ~ ")").close
 
   def plain: PlainPrinter = this
 

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -3777,7 +3777,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                 adaptToSubType(wtp)
           case CompareResult.OKwithGADTUsed
           if pt.isValueType
-             && !inContext(ctx.fresh.setGadt(EmptyGadtConstraint)) {
+             && !inContext(ctx.fresh.setGadt(GadtConstraint.empty)) {
                val res = (tree.tpe.widenExpr frozen_<:< pt)
                if res then
                  // we overshot; a cast is not needed, after all.