Fix #405: Completely overhaul erasure of value classes.

Including when they contain arrays or are contained in arrays.

Author: sjrd

build.sbt

@@ -114,7 +114,11 @@ lazy val tastyQuery =
         import com.typesafe.tools.mima.core.*
         Seq(
           // private, not an issue
+          ProblemFilters.exclude[MissingClassProblem]("tastyquery.Erasure$ErasedValueClass"),
+          ProblemFilters.exclude[MissingClassProblem]("tastyquery.Erasure$ErasedValueClass$"),
           ProblemFilters.exclude[MissingClassProblem]("tastyquery.TypeOps$TypeFold"),
+          // private[tastyquery], not an issue
+          ProblemFilters.exclude[DirectMissingMethodProblem]("tastyquery.Signatures#Signature.toSigName"),
           // Everything in tastyquery.reader is private[tastyquery] at most
           ProblemFilters.exclude[Problem]("tastyquery.reader.*"),
         )

tasty-query/shared/src/main/scala/tastyquery/Definitions.scala

@@ -396,6 +396,15 @@ final class Definitions private[tastyquery] (ctx: Context, rootPackage: PackageS
   lazy val CharClass = scalaPackage.requiredClass("Char")
   lazy val UnitClass = scalaPackage.requiredClass("Unit")
 
+  private[tastyquery] lazy val BoxedBooleanClass = javaLangPackage.requiredClass("Boolean")
+  private[tastyquery] lazy val BoxedCharClass = javaLangPackage.requiredClass("Character")
+  private[tastyquery] lazy val BoxedByteClass = javaLangPackage.requiredClass("Byte")
+  private[tastyquery] lazy val BoxedShortClass = javaLangPackage.requiredClass("Short")
+  private[tastyquery] lazy val BoxedIntClass = javaLangPackage.requiredClass("Integer")
+  private[tastyquery] lazy val BoxedLongClass = javaLangPackage.requiredClass("Long")
+  private[tastyquery] lazy val BoxedFloatClass = javaLangPackage.requiredClass("Float")
+  private[tastyquery] lazy val BoxedDoubleClass = javaLangPackage.requiredClass("Double")
+
   lazy val StringClass = javaLangPackage.requiredClass("String")
 
   lazy val ProductClass = scalaPackage.requiredClass("Product")

tasty-query/shared/src/main/scala/tastyquery/Erasure.scala

@@ -11,10 +11,6 @@ import tastyquery.Types.*
 import tastyquery.Types.ErasedTypeRef.*
 
 private[tastyquery] object Erasure:
-  // TODO: improve this to match dotty:
-  // - use correct type erasure algorithm from Scala 3, with specialisations
-  //   for Java types and Scala 2 types (i.e. varargs, value-classes)
-
   @deprecated("use the overload that takes an explicit SourceLanguage", since = "0.7.1")
   def erase(tpe: Type)(using Context): ErasedTypeRef =
     erase(tpe, SourceLanguage.Scala3)
@@ -27,44 +23,41 @@ private[tastyquery] object Erasure:
     finishErase(preErase(tpe, keepUnit))
   end erase
 
-  /** First pass of erasure, where some special types are preserved as is.
+  private[tastyquery] def eraseForSigName(tpe: Type, language: SourceLanguage, keepUnit: Boolean)(
+    using Context
+  ): ErasedTypeRef =
+    given SourceLanguage = language
+
+    val patchedPreErased = preErase(tpe, keepUnit) match
+      case ArrayTypeRef(ClassRef(cls), dimensions) if cls.isDerivedValueClass =>
+        // Hack! dotc's `sigName` does *not* correspond to erasure in this case!
+        val patchedBase =
+          if cls.typeParams.isEmpty then preEraseMonoValueClass(cls)
+          else preErasePolyValueClass(cls, cls.typeParams.map(_.localRef))
+        patchedBase.underlying.multiArrayOf(dimensions)
+      case typeRef =>
+        typeRef
+
+    finishErase(patchedPreErased)
+  end eraseForSigName
+
+  private final case class ErasedValueClass(valueClass: ClassSymbol, underlying: ErasedTypeRef)
+
+  private type PreErasedTypeRef = ErasedTypeRef | ErasedValueClass
+
+  /** First pass of erasure, where some special types are preserved as is,
+    * and where value classes become `ErasedValueClass`es.
     *
     * In particular, `Any` is preserved as `Any`, instead of becoming
     * `java.lang.Object`.
     */
-  private def preErase(tpe: Type, keepUnit: Boolean)(using Context, SourceLanguage): ErasedTypeRef =
-    def arrayOfBounds(bounds: TypeBounds): ErasedTypeRef =
-      preErase(bounds.high, keepUnit = false) match
-        case ClassRef(cls) if cls.isAny || cls.isAnyVal =>
-          ClassRef(defn.ObjectClass)
-        case typeRef =>
-          typeRef.arrayOf()
-
-    def arrayOf(tpe: TypeOrWildcard): ErasedTypeRef = tpe match
-      case tpe: AppliedType =>
-        tpe.tycon match
-          case TypeRef.OfClass(cls) =>
-            if cls.isArray then
-              val List(targ) = tpe.args: @unchecked
-              arrayOf(targ).arrayOf()
-            else ClassRef(cls).arrayOf()
-          case _ =>
-            arrayOf(tpe.translucentSuperType)
-      case TypeRef.OfClass(cls) =>
-        if cls.isUnit then ClassRef(defn.ErasedBoxedUnitClass).arrayOf()
-        else ClassRef(cls).arrayOf()
-      case tpe: TypeRef =>
-        tpe.optSymbol match
-          case Some(sym: TypeMemberSymbol) if sym.isOpaqueTypeAlias =>
-            arrayOf(tpe.translucentSuperType)
-          case _ =>
-            tpe.bounds match
-              case bounds: AbstractTypeBounds => arrayOfBounds(bounds)
-              case TypeAlias(alias)           => arrayOf(alias)
-      case tpe: TypeParamRef    => arrayOfBounds(tpe.bounds)
-      case tpe: Type            => preErase(tpe, keepUnit = false).arrayOf()
-      case tpe: WildcardTypeArg => arrayOfBounds(tpe.bounds)
-    end arrayOf
+  private def preErase(tpe: Type, keepUnit: Boolean)(using Context, SourceLanguage): PreErasedTypeRef =
+    def arrayOf(tpe: TypeOrWildcard): ErasedTypeRef =
+      if isGenericArrayElement(tpe) then ClassRef(defn.ObjectClass)
+      else
+        preErase(tpe.highIfWildcard, keepUnit = false) match
+          case base: ErasedTypeRef             => base.arrayOf()
+          case ErasedValueClass(valueClass, _) => ClassRef(valueClass).arrayOf()
 
     tpe match
       case tpe: AppliedType =>
@@ -73,11 +66,13 @@ private[tastyquery] object Erasure:
             if cls.isArray then
               val List(targ) = tpe.args: @unchecked
               arrayOf(targ)
+            else if cls.isDerivedValueClass then preErasePolyValueClass(cls, tpe.args)
             else ClassRef(cls)
           case _ =>
             preErase(tpe.translucentSuperType, keepUnit)
       case TypeRef.OfClass(cls) =>
         if !keepUnit && cls.isUnit then ClassRef(defn.ErasedBoxedUnitClass)
+        else if cls.isDerivedValueClass then preEraseMonoValueClass(cls)
         else ClassRef(cls)
       case tpe: TypeRef =>
         preErase(tpe.translucentSuperType, keepUnit)
@@ -90,7 +85,10 @@ private[tastyquery] object Erasure:
           case Some(reduced) => preErase(reduced, keepUnit)
           case None          => preErase(tpe.bound, keepUnit)
       case tpe: OrType =>
-        erasedLub(preErase(tpe.first, keepUnit = false), preErase(tpe.second, keepUnit = false))
+        erasedLub(
+          finishErase(preErase(tpe.first, keepUnit = false)),
+          finishErase(preErase(tpe.second, keepUnit = false))
+        )
       case tpe: AndType =>
         summon[SourceLanguage] match
           case SourceLanguage.Java =>
@@ -120,29 +118,157 @@ private[tastyquery] object Erasure:
         throw IllegalArgumentException(s"Unexpected type in erasure: $tpe")
   end preErase
 
-  private def finishErase(typeRef: ErasedTypeRef)(using Context): ErasedTypeRef =
+  private def finishErase(typeRef: PreErasedTypeRef)(using Context, SourceLanguage): ErasedTypeRef =
     typeRef match
-      case ClassRef(cls) =>
-        if cls.isDerivedValueClass then finishEraseValueClass(cls)
-        else cls.erasure
-      case ArrayTypeRef(ClassRef(cls), dimensions) =>
-        ArrayTypeRef(cls.erasure, dimensions)
+      case ClassRef(cls)                           => cls.erasure
+      case ArrayTypeRef(ClassRef(cls), dimensions) => ArrayTypeRef(cls.erasure, dimensions)
+      case ErasedValueClass(_, underlying)         => finishErase(underlying)
   end finishErase
 
-  private def finishEraseValueClass(cls: ClassSymbol)(using Context): ErasedTypeRef =
+  private def preEraseMonoValueClass(cls: ClassSymbol)(using Context, SourceLanguage): ErasedValueClass =
+    val ctor = cls.findNonOverloadedDecl(nme.Constructor)
+
+    val underlying = ctor.declaredType match
+      case tpe: MethodType if tpe.paramNames.sizeIs == 1 =>
+        tpe.paramTypes.head
+      case _ =>
+        throw InvalidProgramStructureException(s"Illegal value class constructor type ${ctor.declaredType.showBasic}")
+
+    // The underlying of value classes are never value classes themselves (by language spec)
+    val erasedUnderlying = preErase(underlying, keepUnit = false).asInstanceOf[ErasedTypeRef]
+
+    ErasedValueClass(cls, erasedUnderlying)
+  end preEraseMonoValueClass
+
+  private def preErasePolyValueClass(cls: ClassSymbol, targs: List[TypeOrWildcard])(
+    using Context,
+    SourceLanguage
+  ): ErasedValueClass =
     val ctor = cls.findNonOverloadedDecl(nme.Constructor)
 
     def illegalConstructorType(): Nothing =
       throw InvalidProgramStructureException(s"Illegal value class constructor type ${ctor.declaredType.showBasic}")
 
     def ctorParamType(tpe: TypeOrMethodic): Type = tpe match
       case tpe: MethodType if tpe.paramTypes.sizeIs == 1 => tpe.paramTypes.head
-      case tpe: MethodType                               => illegalConstructorType()
-      case tpe: PolyType                                 => ctorParamType(tpe.resultType)
-      case tpe: Type                                     => illegalConstructorType()
+      case _                                             => illegalConstructorType()
+
+    val ctorPolyType = ctor.declaredType match
+      case tpe: PolyType => tpe
+      case _             => illegalConstructorType()
+
+    val genericUnderlying = ctorParamType(ctorPolyType.resultType)
+    val specializedUnderlying = ctorParamType(ctorPolyType.instantiate(targs))
+
+    // The underlying of value classes are never value classes themselves (by language spec)
+    val erasedGenericUnderlying = preErase(genericUnderlying, keepUnit = false).asInstanceOf[ErasedTypeRef]
+    val erasedSpecializedUnderlying = preErase(specializedUnderlying, keepUnit = false).asInstanceOf[ErasedTypeRef]
 
-    erase(ctorParamType(ctor.declaredType), ctor.sourceLanguage)
-  end finishEraseValueClass
+    def isPrimitive(typeRef: ErasedTypeRef): Boolean = typeRef match
+      case ClassRef(cls)   => cls.isPrimitiveValueClass
+      case _: ArrayTypeRef => false
+
+    /* Ideally, we would just use `erasedSpecializedUnderlying` as the erasure of `tp`.
+     * However, there are two special cases for polymorphic value classes, which
+     * historically come from Scala 2:
+     *
+     * - Given `class Foo[A](x: A) extends AnyVal`, `Foo[X]` should erase like
+     *   `X`, except if its a primitive in which case it erases to the boxed
+     *   version of this primitive.
+     * - Given `class Bar[A](x: Array[A]) extends AnyVal`, `Bar[X]` will be
+     *   erased like `Array[A]` as seen from its definition site, no matter
+     *   the `X` (same if `A` is bounded).
+     */
+    val erasedValueClass =
+      if isPrimitive(erasedSpecializedUnderlying) && !isPrimitive(erasedGenericUnderlying) then
+        ClassRef(erasedSpecializedUnderlying.asInstanceOf[ClassRef].cls.boxedClass)
+      else if genericUnderlying.baseType(defn.ArrayClass).isDefined then erasedGenericUnderlying
+      else erasedSpecializedUnderlying
+
+    ErasedValueClass(cls, erasedValueClass)
+  end preErasePolyValueClass
+
+  /** Is `Array[tp]` a generic Array that needs to be erased to `Object`?
+    * This is true if among the subtypes of `Array[tp]` there is either:
+    * - both a reference array type and a primitive array type
+    *   (e.g. `Array[_ <: Int | String]`, `Array[_ <: Any]`)
+    * - or two different primitive array types (e.g. `Array[_ <: Int | Double]`)
+    * In both cases the erased lub of those array types on the JVM is `Object`.
+    *
+    * In addition, if `isScala2` is true, we mimic the Scala 2 erasure rules and
+    * also return true for element types upper-bounded by a non-reference type
+    * such as in `Array[_ <: Int]` or `Array[_ <: UniversalTrait]`.
+    */
+  private def isGenericArrayElement(tp: TypeOrWildcard)(using Context, SourceLanguage): Boolean =
+    /** A symbol that represents the sort of JVM array that values of type `tp` can be stored in:
+      * - If we can always store such values in a reference array, return `j.l.Object`.
+      * - If we can always store them in a specific primitive array, return the corresponding primitive class.
+      * - Otherwise, return `None`.
+      */
+    def arrayUpperBound(tp: Type): Option[ClassSymbol] = tp.dealias match
+      case TypeRef.OfClass(cls) =>
+        def isScala2SpecialCase: Boolean =
+          summon[SourceLanguage] == SourceLanguage.Scala2
+            && !cls.isNull
+            && !cls.isSubClass(defn.ObjectClass)
+
+        // Only a few classes have both primitives and references as subclasses.
+        if cls.isAny || cls.isAnyVal || cls.isMatchable || cls.isSingleton || isScala2SpecialCase then None
+        else if cls.isPrimitiveValueClass then Some(cls)
+        else
+          // Derived value classes in arrays are always boxed, so they end up here as well
+          Some(defn.ObjectClass)
+
+      case tp: TypeProxy =>
+        arrayUpperBound(tp.translucentSuperType)
+      case tp: AndType =>
+        arrayUpperBound(tp.first).orElse(arrayUpperBound(tp.second))
+      case tp: OrType =>
+        val firstBound = arrayUpperBound(tp.first)
+        val secondBound = arrayUpperBound(tp.first)
+        if firstBound == secondBound then firstBound
+        else None
+      case _: NothingType | _: AnyKindType | _: TypeLambda =>
+        None
+      case tp: CustomTransientGroundType =>
+        throw IllegalArgumentException(s"Unexpected transient type: $tp")
+    end arrayUpperBound
+
+    /** Can one of the JVM Array type store all possible values of type `tp`? */
+    def fitsInJVMArray(tp: Type): Boolean = arrayUpperBound(tp).isDefined
+
+    tp match
+      case tp: WildcardTypeArg =>
+        !fitsInJVMArray(tp.bounds.high)
+
+      case tp: Type =>
+        tp.dealias match
+          case tp: TypeRef =>
+            tp.optSymbol match
+              case Some(cls: ClassSymbol) =>
+                false
+              case Some(sym: TypeMemberSymbol) if sym.isOpaqueTypeAlias =>
+                isGenericArrayElement(tp.translucentSuperType)
+              case _ =>
+                tp.bounds match
+                  case TypeAlias(alias)            => isGenericArrayElement(alias)
+                  case AbstractTypeBounds(_, high) => !fitsInJVMArray(high)
+          case tp: TypeParamRef =>
+            !fitsInJVMArray(tp)
+          case tp: MatchType =>
+            val cases = tp.cases
+            cases.nonEmpty && !fitsInJVMArray(cases.map(_.result).reduce(OrType(_, _)))
+          case tp: TypeProxy =>
+            isGenericArrayElement(tp.translucentSuperType)
+          case tp: AndType =>
+            isGenericArrayElement(tp.first) && isGenericArrayElement(tp.second)
+          case tp: OrType =>
+            isGenericArrayElement(tp.first) || isGenericArrayElement(tp.second)
+          case _: NothingType | _: AnyKindType | _: TypeLambda =>
+            false
+          case tp: CustomTransientGroundType =>
+            throw IllegalArgumentException(s"Unexpected transient type: $tp")
+  end isGenericArrayElement
 
   /** The erased least upper bound of two erased types is computed as follows.
     *
@@ -224,7 +350,7 @@ private[tastyquery] object Erasure:
     * - Associativity and commutativity, because this method acts as the minimum
     *   of the total order induced by `compareErasedGlb`.
     */
-  private def erasedGlb(tp1: ErasedTypeRef, tp2: ErasedTypeRef)(using Context): ErasedTypeRef =
+  private def erasedGlb(tp1: PreErasedTypeRef, tp2: PreErasedTypeRef)(using Context): PreErasedTypeRef =
     if compareErasedGlb(tp1, tp2) <= 0 then tp1
     else tp2
 
@@ -248,7 +374,7 @@ private[tastyquery] object Erasure:
     *
     * @see erasedGlb
     */
-  private def compareErasedGlb(tp1: ErasedTypeRef, tp2: ErasedTypeRef)(using Context): Int =
+  private def compareErasedGlb(tp1: PreErasedTypeRef, tp2: PreErasedTypeRef)(using Context): Int =
     def compareClasses(cls1: ClassSymbol, cls2: ClassSymbol): Int =
       if cls1.isSubClass(cls2) then -1
       else if cls2.isSubClass(cls1) then 1
@@ -260,13 +386,11 @@ private[tastyquery] object Erasure:
         // fast path
         0
 
-      case (ClassRef(cls1), _) if cls1.isDerivedValueClass =>
-        tp2 match
-          case ClassRef(cls2) if cls2.isDerivedValueClass =>
-            compareClasses(cls1, cls2)
-          case _ =>
-            -1
-      case (_, ClassRef(cls2)) if cls2.isDerivedValueClass =>
+      case (ErasedValueClass(cls1, _), ErasedValueClass(cls2, _)) =>
+        compareClasses(cls1, cls2)
+      case (ErasedValueClass(cls1, _), _) =>
+        -1
+      case (_, ErasedValueClass(cls2, _)) =>
         1
 
       case (tp1: ArrayTypeRef, tp2: ArrayTypeRef) =>