An alternative scheme for precise apply methods of enums

Keeps many elements from #9922 but the modality where we do the
widening is different. The new rule is as follows:

In an application of a compiler-generated apply or copy method of an enum case,
widen its type to the underlying supertype of the enum case by means of a type
ascription, unless the expected type is an enum case itself.

Author: odersky

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

@@ -658,15 +658,6 @@ 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
@@ -697,8 +688,6 @@ 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 {
@@ -711,9 +700,8 @@ 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, TypeTree(), creatorExpr)
+              .withMods(appMods) :: Nil
           }
         val unapplyMeth = {
           val hasRepeatedParam = constrVparamss.head.exists {

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

@@ -201,52 +201,6 @@ object DesugarEnums {
         TypeTree(), creator).withFlags(Private | Synthetic)
   }
 
-  /** The return type of an enum case apply method and any widening methods in which
-   *  the apply's right hand side will be wrapped. For parents of the form
-   *
-   *      extends E(args) with T1(args1) with ... TN(argsN)
-   *
-   *  and type parameters `tparams` the generated widen method is
-   *
-   *      def C$to$E[tparams](x$1: E[tparams] with T1 with ... TN) = x$1
-   *
-   *  @param cdef            The case definition
-   *  @param parents         The declared parents of the enum case
-   *  @param tparams         The type parameters of the enum case
-   *  @param appliedEnumRef  The enum class applied to `tparams`.
-   */
-  def enumApplyResult(
-      cdef: TypeDef,
-      parents: List[Tree],
-      tparams: List[TypeDef],
-      appliedEnumRef: Tree)(using Context): (Tree, List[DefDef]) = {
-
-    def extractType(t: Tree): Tree = t match {
-      case Apply(t1, _) => extractType(t1)
-      case TypeApply(t1, ts) => AppliedTypeTree(extractType(t1), ts)
-      case Select(t1, nme.CONSTRUCTOR) => extractType(t1)
-      case New(t1) => t1
-      case t1 => t1
-    }
-
-    val parentTypes = parents.map(extractType)
-    parentTypes.head match {
-      case parent: RefTree if parent.name == enumClass.name =>
-        // need a widen method to compute correct type parameters for enum base class
-        val widenParamType = parentTypes.tail.foldLeft(appliedEnumRef)(makeAndType)
-        val widenParam = makeSyntheticParameter(tpt = widenParamType)
-        val widenDef = DefDef(
-          name = s"${cdef.name}$$to$$${enumClass.name}".toTermName,
-          tparams = tparams,
-          vparamss = (widenParam :: Nil) :: Nil,
-          tpt = TypeTree(),
-          rhs = Ident(widenParam.name))
-        (TypeTree(), widenDef :: Nil)
-      case _ =>
-        (parentTypes.reduceLeft(makeAndType), Nil)
-    }
-  }
-
   /** Is a type parameter in `enumTypeParams` referenced from an enum class case that has
    *  given type parameters `caseTypeParams`, value parameters `vparamss` and parents `parents`?
    *  Issues an error if that is the case but the reference is illegal.

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

@@ -160,6 +160,9 @@ object SymUtils {
   def isField(using Context): Boolean =
     self.isTerm && !self.is(Method)
 
+  def isEnumCase(using Context): Boolean =
+    self.isAllOf(EnumCase, butNot = JavaDefined)
+
   def annotationsCarrying(meta: ClassSymbol)(using Context): List[Annotation] =
     self.annotations.filter(_.symbol.hasAnnotation(meta))
 

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

@@ -25,6 +25,7 @@ import ProtoTypes._
 import Inferencing._
 import reporting._
 import transform.TypeUtils._
+import transform.SymUtils._
 import Nullables.{postProcessByNameArgs, given _}
 import config.Feature
 
@@ -891,7 +892,9 @@ trait Applications extends Compatibility {
           case funRef: TermRef =>
             val app = ApplyTo(tree, fun1, funRef, proto, pt)
             convertNewGenericArray(
-              postProcessByNameArgs(funRef, app).computeNullable())
+              widenEnumCase(
+                postProcessByNameArgs(funRef, app).computeNullable(),
+                pt))
           case _ =>
             handleUnexpectedFunType(tree, fun1)
         }
@@ -1091,7 +1094,7 @@ trait Applications extends Compatibility {
    *  It is performed during typer as creation of generic arrays needs a classTag.
    *  we rely on implicit search to find one.
    */
-  def convertNewGenericArray(tree: Tree)(using Context):  Tree = tree match {
+  def convertNewGenericArray(tree: Tree)(using Context): Tree = tree match {
     case Apply(TypeApply(tycon, targs@(targ :: Nil)), args) if tycon.symbol == defn.ArrayConstructor =>
       fullyDefinedType(tree.tpe, "array", tree.span)
 
@@ -1107,6 +1110,26 @@ trait Applications extends Compatibility {
       tree
   }
 
+  /** If `tree` is a complete application of a compiler-generated `apply`
+   *  or `copy` method of an enum case, widen its type to the underlying
+   *  type by means of a type ascription, unless the expected type is an
+   *  enum case itself.
+   *  The underlying type is the intersection of all class parents of the
+   *  orginal type.
+   */
+  def widenEnumCase(tree: Tree, pt: Type)(using Context): Tree =
+    val sym = tree.symbol
+    def isEnumCopy = sym.name == nme.copy && sym.owner.isEnumCase
+    def isEnumApply = sym.name == nme.apply && sym.owner.linkedClass.isEnumCase
+    if sym.is(Synthetic) && (isEnumApply || isEnumCopy)
+       && tree.tpe.classSymbol.isEnumCase
+       && !pt.isInstanceOf[FunProto]
+       && !pt.classSymbol.isEnumCase
+    then
+      Typed(tree, TypeTree(tree.tpe.parents.reduceLeft(TypeComparer.andType(_, _))))
+    else
+      tree
+
   /** Does `state` contain a  "NotAMember" or "MissingIdent" message as
    *  first pending error message? That message would be
    *  `$memberName is not a member of ...` or `Not found: $memberName`.

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

@@ -132,6 +132,9 @@ class ReTyper extends Typer with ReChecking {
   override def inferView(from: Tree, to: Type)(using Context): Implicits.SearchResult =
     Implicits.NoMatchingImplicitsFailure
   override def checkCanEqual(ltp: Type, rtp: Type, span: Span)(using Context): Unit = ()
+
+  override def widenEnumCase(tree: Tree, pt: Type)(using Context): Tree = tree
+
   override protected def addAccessorDefs(cls: Symbol, body: List[Tree])(using Context): List[Tree] = body
   override protected def checkEqualityEvidence(tree: tpd.Tree, pt: Type)(using Context): Unit = ()
   override protected def matchingApply(methType: MethodOrPoly, pt: FunProto)(using Context): Boolean = true

compiler/test-resources/type-printer/enum-precise

@@ -0,0 +1,12 @@
+scala> enum Maybe[+T] { case Something(value: T); case EmptyValue }; import Maybe._
+// defined class Maybe
+
+scala> List(Something(1))
+val res0: List[Maybe[Int]] = List(Something(1))
+
+scala> def listOfSomething[O <: Maybe.Something[_]](listOfSomething: List[O]): listOfSomething.type = listOfSomething
+def listOfSomething
+  [O <: Maybe.Something[?]](listOfSomething: List[O]): listOfSomething.type
+
+scala> listOfSomething(List(Something(1)))
+val res1: List[Maybe.Something[Int]] = List(Something(1))

tests/patmat/i7186.scala

@@ -172,16 +172,16 @@ object printMips {
 
     def oneAddr[O]
       ( a: O, indent: String)
-      ( r: O => Dest,
+      ( r: a.type => Dest,
       ): String = {
         val name = a.getClass.getSimpleName.toLowerCase
         s"${indent}$name ${rsrc(r(a))}$endl"
       }
 
     def twoAddr[O]
       ( a: O, indent: String)
-      ( d: O => Register,
-        r: O => Dest | Constant
+      ( d: a.type => Register,
+        r: a.type => Dest | Constant
       ): String = {
         val name = a.getClass.getSimpleName.toLowerCase
         s"${indent}$name ${registers(d(a))}, ${rsrc(r(a))}$endl"
@@ -190,9 +190,9 @@ object printMips {
     def threeAddr[O]
       ( a: O,
         indent: String )
-      ( d: O => Register,
-        l: O => Register,
-        r: O => Src
+      ( d: a.type => Register,
+        l: a.type => Register,
+        r: a.type => Src
       ): String = {
         val name = a.getClass.getSimpleName.toLowerCase
         s"${indent}$name ${registers(d(a))}, ${registers(l(a))}, ${rsrc(r(a))}$endl"

tests/pos/enum-widen.scala

@@ -0,0 +1,16 @@
+object test:
+
+  enum Option[+T]:
+    case Some[T](x: T) extends Option[T]
+    case None
+
+  import Option._
+
+  var x = Some(1)
+  val y: Some[Int] = Some(2)
+  var xc = y.copy(3)
+  val yc: Some[Int] = y.copy(3)
+  x = None
+  xc = None
+
+

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/enum-nat-macro/Macros_2.scala

@@ -0,0 +1,30 @@
+import Nat._
+
+ inline def toIntMacro(inline nat: Nat): Int = ${ Macros.toIntImpl('nat) }
+ inline def ZeroMacro: Zero.type = ${ Macros.natZero }
+ transparent inline def toNatMacro(inline int: Int): Nat = ${ Macros.toNatImpl('int) }
+
+ object Macros:
+   import quoted._
+
+   def toIntImpl(nat: Expr[Nat])(using QuoteContext): Expr[Int] =
+
+     def inner(nat: Expr[Nat], acc: Int): Int = nat match
+       case '{ Succ($nat) } => inner(nat, acc + 1)
+       case '{ Zero       } => acc
+
+     Expr(inner(nat, 0))
+
+   def natZero(using QuoteContext): Expr[Nat.Zero.type] = '{Zero}
+
+   def toNatImpl(int: Expr[Int])(using QuoteContext): Expr[Nat] =
+
+     // it seems even with the bound that the arg will always widen to Expr[Nat] unless explicit
+
+     def inner[N <: Nat: Type](int: Int, acc: Expr[N]): Expr[Nat] = int match
+       case 0 => acc
+       case n => inner[Succ[N]](n - 1, '{Succ($acc)})
+
+     val Const(i) = int
+     require(i >= 0)
+     inner[Zero.type](i, '{Zero})

tests/run-macros/enum-nat-macro/Nat_1.scala

@@ -0,0 +1,3 @@
+enum Nat:
+  case Zero
+  case Succ[N <: Nat](n: N)

tests/run-macros/enum-nat-macro/Test_3.scala

@@ -0,0 +1,9 @@
+import Nat._
+
+@main def Test: Unit =
+  assert(toIntMacro(Succ(Succ(Succ(Zero)))) == 3)
+  assert(toNatMacro(3) == Succ(Succ(Succ(Zero))))
+  val zero: Zero.type = ZeroMacro
+  assert(zero == Zero)
+  assert(toIntMacro(toNatMacro(3)) == 3)
+  val n: Succ[Succ[Succ[Zero.type]]] = toNatMacro(3)

tests/run-macros/i8007.check

@@ -11,5 +11,7 @@ true
 
 true
 
+true
+
 false
 

tests/run-macros/i8007/Macro_3.scala

@@ -73,7 +73,7 @@ object Eq {
 }
 
 object Macro3 {
-  extension [T](x: =>T) inline def === (y: =>T)(using eq: Eq[T]): Boolean = eq.eqv(x, y)
+  extension [T](inline x: T) inline def === (inline 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

@@ -6,7 +6,12 @@ import Macro3.eqGen
 case class Person(name: String, age: Int)
 
 enum Opt[+T] {
-  case Sm[T](t: T) extends Opt[T]
+  case Sm(t: T)
+  case Nn
+}
+
+enum OptInv[+T] {
+  case Sm[T](t: T) extends OptInv[T]
   case Nn
 }
 
@@ -34,6 +39,11 @@ enum Opt[+T] {
   println(t5) // true
   println
 
+  // Here invariant case without explicit type parameter will instantiate T to OptInv[Any]
+  val t5_2 = OptInv.Sm[Int](23) === OptInv.Sm(23)
+  println(t5) // true
+  println
+
   val t6 = Sm(Person("Test", 23)) === Sm(Person("Test", 23))
   println(t6) // true
   println

tests/run/enums-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)