fix scala#9482: implement manifest algorithm

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

@@ -764,6 +764,12 @@ class Definitions {
   @tu lazy val SelectableClass: ClassSymbol = requiredClass("scala.Selectable")
   @tu lazy val WithoutPreciseParameterTypesClass: Symbol = requiredClass("scala.Selectable.WithoutPreciseParameterTypes")
 
+  @tu lazy val ManifestClass: ClassSymbol = requiredClass("scala.reflect.Manifest")
+  @tu lazy val ManifestFactoryModule: Symbol = requiredModule("scala.reflect.ManifestFactory")
+  @tu lazy val ClassManifestFactoryModule: Symbol = requiredModule("scala.reflect.ClassManifestFactory")
+  @tu lazy val OptManifestClass: ClassSymbol = requiredClass("scala.reflect.OptManifest")
+  @tu lazy val NoManifestModule: Symbol = requiredModule("scala.reflect.NoManifest")
+
   @tu lazy val ReflectPackageClass: Symbol = requiredPackage("scala.reflect.package").moduleClass
   @tu lazy val ClassTagClass: ClassSymbol = requiredClass("scala.reflect.ClassTag")
   @tu lazy val ClassTagModule: Symbol = ClassTagClass.companionModule
@@ -1433,6 +1439,8 @@ class Definitions {
 
   @tu lazy val SpecialClassTagClasses: Set[Symbol] = Set(UnitClass, AnyClass, AnyValClass)
 
+  @tu lazy val SpecialManifestClasses: Set[Symbol] = Set(AnyClass, AnyValClass, ObjectClass, NullClass, NothingClass)
+
   /** Classes that are known not to have an initializer irrespective of
    *  whether NoInits is set. Note: FunctionXXLClass is in this set
    *  because if it is compiled by Scala2, it does not get a NoInit flag.

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

@@ -367,7 +367,6 @@ object StdNames {
     val EnumValue: N            = "EnumValue"
     val ExistentialTypeTree: N  = "ExistentialTypeTree"
     val Flag : N                = "Flag"
-    val floatHash: N            = "floatHash"
     val Ident: N                = "Ident"
     val Import: N               = "Import"
     val Literal: N              = "Literal"
@@ -414,6 +413,7 @@ object StdNames {
     val argv : N                = "argv"
     val arrayClass: N           = "arrayClass"
     val arrayElementClass: N    = "arrayElementClass"
+    val arrayType: N            = "arrayType"
     val arrayValue: N           = "arrayValue"
     val array_apply : N         = "array_apply"
     val array_clone : N         = "array_clone"
@@ -440,6 +440,7 @@ object StdNames {
     val checkInitialized: N     = "checkInitialized"
     val ClassManifestFactory: N = "ClassManifestFactory"
     val classOf: N              = "classOf"
+    val classType: N            = "classType"
     val clone_ : N              = "clone"
     val common: N               = "common"
     val compiletime : N         = "compiletime"
@@ -481,6 +482,7 @@ object StdNames {
     val find_ : N               = "find"
     val flagsFromBits : N       = "flagsFromBits"
     val flatMap: N              = "flatMap"
+    val floatHash: N            = "floatHash"
     val foreach: N              = "foreach"
     val format: N               = "format"
     val fromDigits: N           = "fromDigits"
@@ -626,6 +628,7 @@ object StdNames {
     val values: N               = "values"
     val view_ : N               = "view"
     val wait_ : N               = "wait"
+    val wildcardType: N         = "wildcardType"
     val withFilter: N           = "withFilter"
     val withFilterIfRefutable: N = "withFilterIfRefutable$"
     val WorksheetWrapper: N     = "WorksheetWrapper"

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

@@ -16,6 +16,7 @@ import transform.TypeUtils._
 import transform.SyntheticMembers._
 import util.Property
 import annotation.{tailrec, constructorOnly}
+import collection.mutable
 
 /** Synthesize terms for special classes */
 class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
@@ -375,14 +376,124 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
           synthesizedSumMirror(formal, span)
       case _ => EmptyTree
 
+  private def escapeJavaArray(elemTp: Type)(using Context): Type = elemTp match
+    case JavaArrayType(elemTp1) => defn.ArrayOf(escapeJavaArray(elemTp1))
+    case _ => elemTp
+
+  private enum ManifestKind:
+    case Full, Opt, Clss
+
+    /** The kind that should be used for an array element, if we are `OptManifest` then this
+     *  prevents wildcards arguments of Arrays being converted to `NoManifest`
+     */
+    def arrayElem = if this == Full then this else Clss
+
+  end ManifestKind
+
+  /** Manifest factory that does enough to satisfy the equality semantics for
+   *  - `scala.reflect.OptManifest` (only runtime class is recorded)
+   *  - `scala.reflect.Manifest` (runtime class of arguments are recorded, with wildcard upper bounds wrapped)
+   *
+   *  `toString` may be different.
+   */
+  private def manifestFactoryOf(kind: ManifestKind): SpecialHandler = (formal, span) =>
+    import ManifestKind.*
+
+    /* Creates a tree that calls the factory method called constructor in object scala.reflect.Manifest */
+    def factoryManifest(constructor: TermName, tparg: Type, args: Tree*): Tree =
+      if args.contains(EmptyTree) then
+        EmptyTree
+      else
+        val factory = if kind == Full then defn.ManifestFactoryModule else defn.ClassManifestFactoryModule
+        applyOverloaded(ref(factory), constructor, args.toList, tparg :: Nil, Types.WildcardType).withSpan(span)
+
+    /* Creates a tree representing one of the singleton manifests.*/
+    def singletonManifest(name: TermName) =
+      ref(defn.ManifestFactoryModule).select(name).ensureApplied.withSpan(span)
+
+    def synthArrayManifest(elemTp: Type, kind: ManifestKind, topLevel: Boolean): Tree =
+      factoryManifest(nme.arrayType, elemTp, synthesize(elemTp, kind.arrayElem, topLevel))
+
+    /** manifests generated from wildcards can not equal Int,Long,Any,AnyRef,AnyVal etc,
+     *  so we wrap their upper bound.
+     */
+    def synthWildcardManifest(tp: Manifestable, hi: Type): Tree =
+      factoryManifest(nme.wildcardType, tp, singletonManifest(nme.Nothing), synthesizeNested(hi))
+
+
+    /** `Nil` if not full manifest */
+    def synthArgManifests(tp: Manifestable): List[Tree] = tp match
+      case AppliedType(_, args) if kind == Full && tp.typeSymbol.isClass =>
+        args.map(synthesizeNested)
+      case _ =>
+        Nil
+
+    /** This type contains all top-level types supported by Scala 2's algorithm.
+     *  This is strictly less than what is supported by `ClassTag`,
+     *  e.g. in Scala 2 `manifest[Int @unchecked]` will fail,
+     *  but `classTag[Int @unchecked]` succeeds.
+     */
+    type Manifestable =
+      ThisType | TermRef | ConstantType | TypeRef | AppliedType | TypeBounds | RecType | RefinedType | AndType
+
+    /** adapted from `syntheticClassTag` */
+    def synthManifest(tp: Manifestable, kind: ManifestKind, topLevel: Boolean) = tp match
+      case defn.ArrayOf(elemTp)              => synthArrayManifest(elemTp, kind, topLevel)
+      case TypeBounds(_, hi) if kind == Full => synthWildcardManifest(tp, hi)
+
+      case tp if hasStableErasure(tp) && !(topLevel && defn.isBottomClassAfterErasure(tp.typeSymbol)) =>
+        // should this be Scala 2 erasure? (e.g. intersection types behave differently)
+        erasure(tp) match
+          // erase first so that union types collapse to one of the singleton Manifests
+          case JavaArrayType(elemTp) =>
+            synthArrayManifest(escapeJavaArray(elemTp), kind, topLevel)
+
+          case etp =>
+            val sym = etp.typeSymbol
+            if sym.isPrimitiveValueClass || defn.SpecialManifestClasses.contains(sym) then
+              singletonManifest(sym.name.toTermName)
+            else
+              val clsArg = clsOf(etp).asInstance(defn.ClassType(tp)) // cast needed to resolve overloading
+              factoryManifest(nme.classType, tp, (clsArg :: synthArgManifests(tp))*)
+
+      case _ =>
+        EmptyTree
+
+    end synthManifest
+
+    def manifestOfType(tp0: Type, kind: ManifestKind, topLevel: Boolean): Tree = tp0.dealiasKeepAnnots match
+      case tp1: Manifestable => synthManifest(tp1, kind, topLevel)
+      case tp1               => EmptyTree
+
+    def synthesize(tp: Type, kind: ManifestKind, topLevel: Boolean = true): Tree =
+      manifestOfType(tp, kind, topLevel) match
+        case EmptyTree if kind == Opt => ref(defn.NoManifestModule)
+        case result                   => result
+
+    def synthesizeNested(tp: Type): Tree = synthesize(tp, Full, topLevel = false)
+
+    formal.argInfos match
+      case arg :: Nil =>
+        synthesize(fullyDefinedType(arg, "Manifest argument", span), kind)
+      case _ =>
+        EmptyTree
+
+  end manifestFactoryOf
+
+  val synthesizedManifest: SpecialHandler = manifestFactoryOf(ManifestKind.Full)
+  val synthesizedOptManifest: SpecialHandler = manifestFactoryOf(ManifestKind.Opt)
+
   val specialHandlers = List(
     defn.ClassTagClass        -> synthesizedClassTag,
     defn.TypeTestClass        -> synthesizedTypeTest,
     defn.CanEqualClass        -> synthesizedCanEqual,
     defn.ValueOfClass         -> synthesizedValueOf,
     defn.Mirror_ProductClass  -> synthesizedProductMirror,
     defn.Mirror_SumClass      -> synthesizedSumMirror,
-    defn.MirrorClass          -> synthesizedMirror)
+    defn.MirrorClass          -> synthesizedMirror,
+    defn.ManifestClass        -> synthesizedManifest,
+    defn.OptManifestClass     -> synthesizedOptManifest,
+  )
 
   def tryAll(formal: Type, span: Span)(using Context): Tree =
     def recur(handlers: SpecialHandlers): Tree = handlers match

tests/pos/i9482.scala

@@ -0,0 +1,11 @@
+import scala.reflect.OptManifest
+
+object Ref {
+  def make[A: OptManifest]: Ref[A] = ???
+}
+trait Ref[A]
+
+trait Foo[A] {
+  val bar = Ref.make[Int]
+  val baz: Ref[A] = Ref.make
+}