Precise apply for enum companion objects

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

@@ -527,6 +527,12 @@ object desugar {
     // a reference to the class type bound by `cdef`, with type parameters coming from the constructor
     val classTypeRef = appliedRef(classTycon)
 
+    def applyResultTpt =
+      if isEnumCase then
+        classTypeRef
+      else
+        TypeTree()
+
     // a reference to `enumClass`, with type parameters coming from the case constructor
     lazy val enumClassTypeRef =
       if (enumClass.typeParams.isEmpty)
@@ -605,7 +611,7 @@ object desugar {
             cpy.ValDef(vparam)(rhs = copyDefault(vparam)))
           val copyRestParamss = derivedVparamss.tail.nestedMap(vparam =>
             cpy.ValDef(vparam)(rhs = EmptyTree))
-          DefDef(nme.copy, derivedTparams, copyFirstParams :: copyRestParamss, TypeTree(), creatorExpr)
+          DefDef(nme.copy, derivedTparams, copyFirstParams :: copyRestParamss, applyResultTpt, creatorExpr)
             .withMods(Modifiers(Synthetic | constr1.mods.flags & copiedAccessFlags, constr1.mods.privateWithin)) :: Nil
         }
       }
@@ -656,15 +662,6 @@ object desugar {
     // For all other classes, the parent is AnyRef.
     val companions =
       if (isCaseClass) {
-        // The return type of the `apply` method, and an (empty or singleton) list
-        // of widening coercions
-        val (applyResultTpt, widenDefs) =
-          if (!isEnumCase)
-            (TypeTree(), Nil)
-          else if (parents.isEmpty || enumClass.typeParams.isEmpty)
-            (enumClassTypeRef, Nil)
-          else
-            enumApplyResult(cdef, parents, derivedEnumParams, appliedRef(enumClassRef, derivedEnumParams))
 
         // true if access to the apply method has to be restricted
         // i.e. if the case class constructor is either private or qualified private
@@ -695,8 +692,6 @@ object desugar {
           then anyRef
           else
             constrVparamss.foldRight(classTypeRef)((vparams, restpe) => Function(vparams map (_.tpt), restpe))
-        def widenedCreatorExpr =
-          widenDefs.foldLeft(creatorExpr)((rhs, meth) => Apply(Ident(meth.name), rhs :: Nil))
         val applyMeths =
           if (mods.is(Abstract)) Nil
           else {
@@ -709,9 +704,8 @@ object desugar {
             val appParamss =
               derivedVparamss.nestedZipWithConserve(constrVparamss)((ap, cp) =>
                 ap.withMods(ap.mods | (cp.mods.flags & HasDefault)))
-            val app = DefDef(nme.apply, derivedTparams, appParamss, applyResultTpt, widenedCreatorExpr)
-              .withMods(appMods)
-            app :: widenDefs
+            DefDef(nme.apply, derivedTparams, appParamss, applyResultTpt, creatorExpr)
+              .withMods(appMods) :: Nil
           }
         val unapplyMeth = {
           val hasRepeatedParam = constrVparamss.head.exists {
@@ -720,7 +714,7 @@ object desugar {
           val methName = if (hasRepeatedParam) nme.unapplySeq else nme.unapply
           val unapplyParam = makeSyntheticParameter(tpt = classTypeRef)
           val unapplyRHS = if (arity == 0) Literal(Constant(true)) else Ident(unapplyParam.name)
-          val unapplyResTp = if (arity == 0) Literal(Constant(true)) else TypeTree()
+          val unapplyResTp = if (arity == 0) Literal(Constant(true)) else applyResultTpt
           DefDef(methName, derivedTparams, (unapplyParam :: Nil) :: Nil, unapplyResTp, unapplyRHS)
             .withMods(synthetic)
         }

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

@@ -1180,6 +1180,8 @@ object Types {
     def widenSingletons(using Context): Type = dealias match {
       case tp: SingletonType =>
         tp.widen
+      case tp: (TypeRef | AppliedType) if tp.typeSymbol.isAllOf(EnumCase) =>
+        tp.parents.head
       case tp: OrType =>
         val tp1w = tp.widenSingletons
         if (tp1w eq tp) this else tp1w

compiler/src/dotty/tools/dotc/parsing/Scanners.scala

@@ -1399,8 +1399,8 @@ object Scanners {
 
   object IndentWidth {
     private inline val MaxCached = 40
-    private val spaces = Array.tabulate(MaxCached + 1)(new Run(' ', _))
-    private val tabs = Array.tabulate(MaxCached + 1)(new Run('\t', _))
+    private val spaces = Array.tabulate[Run](MaxCached + 1)(new Run(' ', _)) // TODO: remove new after bootstrap
+    private val tabs = Array.tabulate[Run](MaxCached + 1)(new Run('\t', _)) // TODO: remove new after bootstrap
 
     def Run(ch: Char, n: Int): Run =
       if (n <= MaxCached && ch == ' ') spaces(n)

tests/pos/i3935.scala

@@ -0,0 +1,10 @@
+enum Foo3[T](x: T) {
+  case Bar[S, T](y: T) extends Foo3[y.type](y)
+}
+
+val foo: Foo3.Bar[Nothing, 3] = Foo3.Bar(3)
+val bar = foo
+
+def baz[T](f: Foo3[T]): f.type = f
+
+val qux = baz(bar) // existentials are back in Dotty?

tests/run-macros/i8007/Macro_3.scala

@@ -64,7 +64,6 @@ object Eq {
               $ordx == $ordy && $elements($ordx).asInstanceOf[Eq[Any]].eqv($x, $y)
           }
         }
-
         '{
           eqSum((x: T, y: T) => ${eqSumBody('x, 'y)})
         }
@@ -76,4 +75,4 @@ object Macro3 {
   extension [T](x: =>T) inline def === (y: =>T)(using eq: Eq[T]): Boolean = eq.eqv(x, y)
 
   implicit inline def eqGen[T]: Eq[T] = ${ Eq.derived[T] }
-}
+}

tests/run-macros/i8007/Test_4.scala

@@ -6,10 +6,22 @@ import Macro3.eqGen
 case class Person(name: String, age: Int)
 
 enum Opt[+T] {
-  case Sm(t: T)
+  case Sm[U](t: U) extends Opt[U]
   case Nn
 }
 
+enum OptInfer[+T] {
+  case Sm[+U](t: U) extends OptInfer[U]
+  case Nn
+}
+
+// simulation of Opt using case class hierarchy
+sealed abstract class OptCase[+T]
+object OptCase {
+  final case class Sm[T](t: T) extends OptCase[T]
+  case object Nn extends OptCase[Nothing]
+}
+
 @main def Test() = {
   import Opt._
   import Eq.{given _, _}
@@ -30,15 +42,23 @@ enum Opt[+T] {
   println(t4) // false
   println
 
-  val t5 = Sm(23) === Sm(23)
+  val t5 = Opt.Sm[Int](23) === Opt.Sm(23) // same behaviour as case class when using apply
   println(t5) // true
   println
 
-  val t6 = Sm(Person("Test", 23)) === Sm(Person("Test", 23))
+  val t5_2 = OptCase.Sm[Int](23) === OptCase.Sm(23)
+  println(t5_2) // true
+  println
+
+  val t5_3 = OptInfer.Sm(23) === OptInfer.Sm(23) // covariant `Sm` case means we can avoid explicit type parameter
+  println(t5_3) // true
+  println
+
+  val t6 = Sm[Person](Person("Test", 23)) === Sm(Person("Test", 23))
   println(t6) // true
   println
 
-  val t7 = Sm(Person("Test", 23)) === Sm(Person("Test", 24))
+  val t7 = Sm[Person](Person("Test", 23)) === Sm(Person("Test", 24))
   println(t7) // false
   println
-}
+}

tests/run/enum-precise.scala

@@ -0,0 +1,31 @@
+enum NonEmptyList[+T]:
+  case Many[+U](head: U, tail: NonEmptyList[U]) extends NonEmptyList[U]
+  case One [+U](value: U)                       extends NonEmptyList[U]
+
+enum Ast:
+  case Binding(name: String, tpe: String)
+  case Lambda(args: NonEmptyList[Binding], rhs: Ast) // reference to another case of the enum
+  case Ident(name: String)
+  case Apply(fn: Ast, args: NonEmptyList[Ast])
+
+import NonEmptyList._
+import Ast._
+
+// This example showcases the widening when inferring enum case types.
+// With scala 2 case class hierarchies, if One.apply(1) returns One[Int] and Many.apply(2, One(3)) returns Many[Int]
+// then the `foldRight` expression below would complain that Many[Binding] is not One[Binding]. With Scala 3 enums,
+// .apply on the companion returns the precise class, but type inference will widen to NonEmptyList[Binding] unless
+// the precise class is expected.
+def Bindings(arg: (String, String), args: (String, String)*): NonEmptyList[Binding] =
+  def Bind(arg: (String, String)): Binding =
+    val (name, tpe) = arg
+    Binding(name, tpe)
+
+  args.foldRight(One[Binding](Bind(arg)))((arg, acc) => Many(Bind(arg), acc))
+
+@main def Test: Unit =
+  val OneOfOne: One[1] = One[1](1)
+  val True = Lambda(Bindings("x" -> "T", "y" -> "T"), Ident("x"))
+  val Const = Lambda(One(Binding("x", "T")), Lambda(One(Binding("y", "U")), Ident("x"))) // precise type is forwarded
+
+  assert(OneOfOne.value == 1)