Fix #10217: Avoid exponential behavior in derivesFrom

compiler/src/dotty/tools/dotc/ast/tpd.scala

@@ -586,7 +586,7 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
           val tree2: Select = tree.tpe match {
             case tpe: NamedType =>
               val qualType = qualifier.tpe.widenIfUnstable
-              if qualType.isNothing then tree1.withTypeUnchecked(tree.tpe)
+              if qualType.isExactlyNothing then tree1.withTypeUnchecked(tree.tpe)
               else tree1.withType(tpe.derivedSelect(qualType))
             case _ => tree1.withTypeUnchecked(tree.tpe)
           }
@@ -980,7 +980,7 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
 
     /** `tree ne null` (might need a cast to be type correct) */
     def testNotNull(using Context): Tree = {
-      val receiver = if (defn.isBottomType(tree.tpe))
+      val receiver = if (tree.tpe.isBottomType)
         // If the receiver is of type `Nothing` or `Null`, add an ascription so that the selection
         // succeeds: e.g. `null.ne(null)` doesn't type, but `(null: AnyRef).ne(null)` does.
         Typed(tree, TypeTree(defn.AnyRefType))

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

@@ -1174,13 +1174,6 @@ class Definitions {
 
   def isBottomClassAfterErasure(cls: Symbol): Boolean = cls == NothingClass || cls == NullClass
 
-  def isBottomType(tp: Type): Boolean =
-    if (ctx.explicitNulls && !ctx.phase.erasedTypes) tp.derivesFrom(NothingClass)
-    else isBottomTypeAfterErasure(tp)
-
-  def isBottomTypeAfterErasure(tp: Type): Boolean =
-    tp.derivesFrom(NothingClass) || tp.derivesFrom(NullClass)
-
   /** Is a function class.
    *   - FunctionXXL
    *   - FunctionN for N >= 0

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

@@ -407,8 +407,8 @@ class TypeApplications(val self: Type) extends AnyVal {
       if (self.derivesFrom(from)) {
         def elemType(tp: Type): Type = tp.widenDealias match
           case tp: OrType =>
-            if defn.isBottomType(tp.tp1) then elemType(tp.tp2)
-            else if defn.isBottomType(tp.tp2) then elemType(tp.tp1)
+            if tp.tp1.isBottomType then elemType(tp.tp2)
+            else if tp.tp2.isBottomType then elemType(tp.tp1)
             else tp.derivedOrType(elemType(tp.tp1), elemType(tp.tp2))
           case tp: AndType => tp.derivedAndType(elemType(tp.tp1), elemType(tp.tp2))
           case _ => tp.baseType(from).argInfos.headOption.getOrElse(defn.NothingType)
@@ -503,8 +503,8 @@ class TypeApplications(val self: Type) extends AnyVal {
   def elemType(using Context): Type = self.widenDealias match {
     case defn.ArrayOf(elemtp) => elemtp
     case JavaArrayType(elemtp) => elemtp
-    case tp: OrType if defn.isBottomType(tp.tp1) => tp.tp2.elemType
-    case tp: OrType if defn.isBottomType(tp.tp2) => tp.tp1.elemType
+    case tp: OrType if tp.tp1.isBottomType => tp.tp2.elemType
+    case tp: OrType if tp.tp2.isBottomType => tp.tp1.elemType
     case _ => self.baseType(defn.SeqClass).argInfos.headOption.getOrElse(NoType)
   }
 }

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

@@ -715,7 +715,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         }
         compareTypeBounds
       case tp2: AnnotatedType if tp2.isRefining =>
-        (tp1.derivesAnnotWith(tp2.annot.sameAnnotation) || defn.isBottomType(tp1)) &&
+        (tp1.derivesAnnotWith(tp2.annot.sameAnnotation) || tp1.isBottomType) &&
         recur(tp1, tp2.parent)
       case ClassInfo(pre2, cls2, _, _, _) =>
         def compareClassInfo = tp1 match {
@@ -2338,7 +2338,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
    */
   def provablyEmpty(tp: Type): Boolean =
     tp.dealias match {
-      case tp if tp.isNothing => true
+      case tp if tp.isExactlyNothing => true
       case AndType(tp1, tp2) => provablyDisjoint(tp1, tp2)
       case OrType(tp1, tp2) => provablyEmpty(tp1) && provablyEmpty(tp2)
       case at @ AppliedType(tycon, args) =>

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

@@ -291,8 +291,8 @@ object TypeErasure {
     // We need to short-circuit this case here because the regular lub logic below
     // relies on the class hierarchy, which doesn't properly capture `Null`s subtyping
     // behaviour.
-    if (defn.isBottomTypeAfterErasure(tp1) && tp2.derivesFrom(defn.ObjectClass)) return tp2
-    if (defn.isBottomTypeAfterErasure(tp2) && tp1.derivesFrom(defn.ObjectClass)) return tp1
+    if (tp1.isBottomTypeAfterErasure && tp2.derivesFrom(defn.ObjectClass)) return tp2
+    if (tp2.isBottomTypeAfterErasure && tp1.derivesFrom(defn.ObjectClass)) return tp1
     tp1 match {
       case JavaArrayType(elem1) =>
         import dotty.tools.dotc.transform.TypeUtils._

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

@@ -152,7 +152,7 @@ object TypeOps:
         simplify(l, theMap) & simplify(r, theMap)
       case tp as OrType(l, r)
       if !ctx.mode.is(Mode.Type)
-         && (tp.isSoft || defn.isBottomType(l) || defn.isBottomType(r)) =>
+         && (tp.isSoft || l.isBottomType || r.isBottomType) =>
         // Normalize A | Null and Null | A to A even if the union is hard (i.e.
         // explicitly declared), but not if -Yexplicit-nulls is set. The reason is
         // that in this case the normal asSeenFrom machinery is not prepared to deal

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

@@ -209,7 +209,27 @@ object Types {
     def isAnyRef(using Context): Boolean  = isRef(defn.ObjectClass, skipRefined = false)
     def isAnyKind(using Context): Boolean = isRef(defn.AnyKindClass, skipRefined = false)
 
-    def isFromJavaObject(using Context): Boolean = typeSymbol eq defn.FromJavaObjectSymbol
+    /** Is this type exactly Nothing (no vars, aliases, refinements etc allowed)? */
+    def isExactlyNothing(using Context): Boolean = this match {
+      case tp: TypeRef =>
+        tp.name == tpnme.Nothing && (tp.symbol eq defn.NothingClass)
+      case _ => false
+    }
+
+    /** Is this type exactly Any (no vars, aliases, refinements etc allowed)? */
+    def isExactlyAny(using Context): Boolean = this match {
+      case tp: TypeRef =>
+        tp.name == tpnme.Any && (tp.symbol eq defn.AnyClass)
+      case _ => false
+    }
+
+    def isBottomType(using Context): Boolean =
+      if ctx.explicitNulls && !ctx.phase.erasedTypes then hasClassSymbol(defn.NothingClass)
+      else isBottomTypeAfterErasure
+
+    def isBottomTypeAfterErasure(using Context): Boolean =
+      val d = defn
+      hasClassSymbol(d.NothingClass) || hasClassSymbol(d.NullClass)
 
     /** Does this type refer exactly to class symbol `sym`, instead of to a subclass of `sym`?
      *  Implemented like `isRef`, but follows more types: all type proxies as well as and- and or-types
@@ -224,7 +244,9 @@ object Types {
       case _ => false
     }
 
-    /** Is this type an instance of a non-bottom subclass of the given class `cls`? */
+    /** True if this type is an instance of the given `cls` or an instance of
+     *  a non-bottom subclass of `cls`.
+     */
     final def derivesFrom(cls: Symbol)(using Context): Boolean = {
       def loop(tp: Type): Boolean = tp match {
         case tp: TypeRef =>
@@ -239,11 +261,15 @@ object Types {
         case tp: AndType =>
           loop(tp.tp1) || loop(tp.tp2)
         case tp: OrType =>
-          // If the type is `T | Null` or `T | Nothing`, and `T` derivesFrom the class,
-          // then the OrType derivesFrom the class. Otherwise, we need to check both sides
-          // derivesFrom the class.
-          if defn.isBottomType(tp.tp1) then loop(tp.tp2)
-          else loop(tp.tp1) && (defn.isBottomType(tp.tp2) || loop(tp.tp2))
+          // If the type is `T | Null` or `T | Nothing`, the class is != Nothing,
+          // and `T` derivesFrom the class, then the OrType derivesFrom the class.
+          // Otherwise, we need to check both sides derivesFrom the class.
+          if tp.tp1.isBottomType && cls != defn.NothingClass then
+            loop(tp.tp2)
+          else if tp.tp2.isBottomType && cls != defn.NothingClass then
+            loop(tp.tp1)
+          else
+            loop(tp.tp1) && loop(tp.tp2)
         case tp: JavaArrayType =>
           cls == defn.ObjectClass
         case _ =>
@@ -252,6 +278,8 @@ object Types {
       loop(this)
     }
 
+    def isFromJavaObject(using Context): Boolean = typeSymbol eq defn.FromJavaObjectSymbol
+
     /** True iff `symd` is a denotation of a class type parameter and the reference
      *  `<this> . <symd>` is an actual argument reference, i.e. `this` is different
      *  from the ThisType of `symd`'s owner.
@@ -265,20 +293,6 @@ object Types {
         }
       }
 
-    /** Is this type exactly Nothing (no vars, aliases, refinements etc allowed)? */
-    def isNothing(using Context): Boolean = this match {
-      case tp: TypeRef =>
-        tp.name == tpnme.Nothing && (tp.symbol eq defn.NothingClass)
-      case _ => false
-    }
-
-    /** Is this type exactly Any (no vars, aliases, refinements etc allowed)? */
-    def isTopType(using Context): Boolean = this match {
-      case tp: TypeRef =>
-        tp.name == tpnme.Any && (tp.symbol eq defn.AnyClass)
-      case _ => false
-    }
-
     /** Is this type a (possibly aliased) singleton type? */
     def isSingleton(using Context): Boolean = dealias.isInstanceOf[SingletonType]
 
@@ -478,6 +492,22 @@ object Types {
     final def classSymbols(using Context): List[ClassSymbol] =
       parentSymbols(_.isClass).asInstanceOf
 
+    /** Same as `this.classSymbols.contains(cls)` but more efficient */
+    final def hasClassSymbol(cls: Symbol)(using Context): Boolean = this match
+      case tp: TypeRef   =>
+        val sym = tp.symbol
+        sym == cls || !sym.isClass && tp.superType.hasClassSymbol(cls)
+      case tp: TypeProxy =>
+        tp.underlying.hasClassSymbol(cls)
+      case tp: ClassInfo =>
+        tp.cls == cls
+      case AndType(l, r) =>
+        l.hasClassSymbol(cls) || r.hasClassSymbol(cls)
+      case OrType(l, r) =>
+        l.hasClassSymbol(cls) && r.hasClassSymbol(cls)
+      case _ =>
+        false
+
     /** The term symbol associated with the type */
     @tailrec final def termSymbol(using Context): Symbol = this match {
       case tp: TermRef => tp.symbol
@@ -2134,8 +2164,8 @@ object Types {
           case arg: TypeBounds =>
             val v = param.paramVarianceSign
             val pbounds = param.paramInfo
-            if (v > 0 && pbounds.loBound.dealiasKeepAnnots.isNothing) TypeAlias(arg.hiBound & rebase(pbounds.hiBound))
-            else if (v < 0 && pbounds.hiBound.dealiasKeepAnnots.isTopType) TypeAlias(arg.loBound | rebase(pbounds.loBound))
+            if (v > 0 && pbounds.loBound.dealiasKeepAnnots.isExactlyNothing) TypeAlias(arg.hiBound & rebase(pbounds.hiBound))
+            else if (v < 0 && pbounds.hiBound.dealiasKeepAnnots.isExactlyAny) TypeAlias(arg.loBound | rebase(pbounds.loBound))
             else arg recoverable_& rebase(pbounds)
           case arg => TypeAlias(arg)
         }
@@ -2297,7 +2327,7 @@ object Types {
           if (base.isAnd == variance >= 0) tp1 & tp2 else tp1 | tp2
         case _ =>
           if (pre.termSymbol.is(Package)) argForParam(pre.select(nme.PACKAGE))
-          else if (pre.isNothing) pre
+          else if (pre.isExactlyNothing) pre
           else NoType
       }
     }
@@ -2316,7 +2346,7 @@ object Types {
      */
     def derivedSelect(prefix: Type)(using Context): Type =
       if (prefix eq this.prefix) this
-      else if (prefix.isNothing) prefix
+      else if (prefix.isExactlyNothing) prefix
       else {
         if (isType) {
           val res =
@@ -4304,7 +4334,7 @@ object Types {
 
     /** For uninstantiated type variables: Is the lower bound different from Nothing? */
     def hasLowerBound(using Context): Boolean =
-      !ctx.typerState.constraint.entry(origin).loBound.isNothing
+      !ctx.typerState.constraint.entry(origin).loBound.isExactlyNothing
 
     /** For uninstantiated type variables: Is the upper bound different from Any? */
     def hasUpperBound(using Context): Boolean =
@@ -5309,7 +5339,7 @@ object Types {
         case _ =>
           def propagate(lo: Type, hi: Type) =
             range(derivedRefinedType(tp, parent, lo), derivedRefinedType(tp, parent, hi))
-          if (parent.isNothing) parent
+          if (parent.isExactlyNothing) parent
           else info match {
             case Range(infoLo: TypeBounds, infoHi: TypeBounds) =>
               assert(variance == 0)
@@ -5402,7 +5432,7 @@ object Types {
         case Range(lo, hi) =>
           range(tp.derivedAnnotatedType(lo, annot), tp.derivedAnnotatedType(hi, annot))
         case _ =>
-          if (underlying.isNothing) underlying
+          if (underlying.isExactlyNothing) underlying
           else tp.derivedAnnotatedType(underlying, annot)
       }
     override protected def derivedWildcardType(tp: WildcardType, bounds: Type): WildcardType =
@@ -5650,7 +5680,7 @@ object Types {
       else {
         seen += tp
         tp match {
-          case tp if tp.isTopType || tp.isNothing =>
+          case tp if tp.isExactlyAny || tp.isExactlyNothing =>
             cs
           case tp: AppliedType =>
             foldOver(cs + tp.typeSymbol, tp)

compiler/src/dotty/tools/dotc/interactive/Completion.scala

@@ -205,7 +205,7 @@ object Completion {
      * considered.
      */
     def addMemberCompletions(qual: Tree)(using Context): Unit =
-      if (!qual.tpe.widenDealias.isNothing) {
+      if (!qual.tpe.widenDealias.isExactlyNothing) {
         addAccessibleMembers(qual.tpe)
         if (!mode.is(Mode.Import) && !qual.tpe.isNullType)
           // Implicit conversions do not kick in when importing

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -269,7 +269,7 @@ import transform.SymUtils._
           if expected.isAny
              || expected.isAnyRef
              || expected.isRef(defn.AnyValClass)
-             || defn.isBottomType(found)
+             || found.isBottomType
           then ""
           else ctx.typer.importSuggestionAddendum(ViewProto(found.widen, expected))
       val (where, printCtx) = Formatting.disambiguateTypes(found2, expected2)

compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -373,7 +373,7 @@ object PatternMatcher {
               patternPlan(casted, pat, onSuccess)
             })
         case UnApply(extractor, implicits, args) =>
-          val unappPlan = if (defn.isBottomType(scrutinee.info))
+          val unappPlan = if (scrutinee.info.isBottomType)
             // Generate a throwaway but type-correct plan.
             // This plan will never execute because it'll be guarded by a `NonNullTest`.
             ResultPlan(tpd.Throw(tpd.nullLiteral))

compiler/src/dotty/tools/dotc/transform/TransformByNameApply.scala

@@ -42,7 +42,7 @@ abstract class TransformByNameApply extends MiniPhase { thisPhase: DenotTransfor
       def transformArg(arg: Tree, formal: Type): Tree = formal.dealias match {
         case formalExpr: ExprType =>
           var argType = arg.tpe.widenIfUnstable
-          if (defn.isBottomType(argType)) argType = formal.widenExpr
+          if (argType.isBottomType) argType = formal.widenExpr
           def wrap(arg: Tree) =
             ref(defn.cbnArg).appliedToType(argType).appliedTo(arg).withSpan(arg.span)
           arg match {

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

@@ -165,7 +165,7 @@ object ErrorReporting {
            |Note that `${tree.name}` is treated as an infix operator in Scala 3.
            |If you do not want that, insert a `;` or empty line in front
            |or drop any spaces behind the operator."""
-      else if qualType.isNothing then
+      else if qualType.isExactlyNothing then
         ""
       else
         val add = suggestImports(

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

@@ -715,7 +715,7 @@ trait ImplicitRunInfo:
               seen += t
               t.underlying match
                 case TypeBounds(lo, hi) =>
-                  if defn.isBottomTypeAfterErasure(lo) then apply(hi)
+                  if lo.isBottomTypeAfterErasure then apply(hi)
                   else AndType.make(apply(lo), apply(hi))
                 case u => apply(u)
 

tests/pos/i10217.scala

@@ -0,0 +1,32 @@
+trait A
+trait B
+trait C
+trait D
+trait E
+trait F
+trait G
+trait H
+trait I
+trait J
+trait K
+trait L
+trait M
+trait N
+trait O
+trait P
+trait Q
+trait R
+trait S
+trait T
+trait U
+trait V
+trait W
+
+class Foo[T]
+
+val f1 = Foo[A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U]
+val f2 = Foo[A | (B | (C | (D | (E | (F | (G | (H | (I | (J | (K | (L | (M | (N | (O | (P | (Q | (R | (S | (T | U)))))))))))))))))))]
+val f3 = Foo[Null | Null | Null | Null | Null | Null | Null | Null | Null | Null | Null | Null | Null | Null | Null | Null | Null | Null | Null | Null | A]
+val f4 = Foo[A | Null | Null | Null | Null | Null | Null | Null | Null | Null | Null | Null | Null | Null | Null | Null | Null | Null | Null | Null | Null]
+val f5 = Foo[Null | (Null | (Null | (Null | (Null | (Null | (Null | (Null | (Null | (Null | (Null | (Null | (Null | (Null | (Null | (Null | (Null | (Null | (Null | (Null | A)))))))))))))))))))]
+val f6 = Foo[A | (Null | (Null | (Null | (Null | (Null | (Null | (Null | (Null | (Null | (Null | (Null | (Null | (Null | (Null | (Null | (Null | (Null | (Null | Null))))))))))))))))))]