Minimal support for dependent case classes (#21698)

This lets us write:

```scala
    trait A:
      type B

    case class CC(a: A, b: a.B)
```

Pattern matching works but isn't dependent yet:

```scala
    x match
      case CC(a, b) =>
        val a1: A = a
        // Dependent pattern matching is not currently supported
        // val b1: a1.B = b
        val b1 = b // Type is CC#a.B
```

(for my usecase this isn't a problem, I'm working on a type constraint
API which lets me write things like `case class CC(a: Int, b: Int
GreaterThan[a.type])`)

Because case class pattern matching relies on the product selectors
`_N`, making it dependent is a bit tricky, currently we generate:

```scala
    case class CC(a: A, b: a.B):
      def _1: A = a
      def _2: a.B = b
```

So the type of `_2` is not obviously related to the type of `_1`, we
probably need to change what we generate into:

```scala
    case class CC(a: A, b: a.B):
      @uncheckedStable def _1: a.type = a
      def _2: _1.B = b
```

But this can be done in a separate PR.

Fixes #8073.

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

@@ -618,6 +618,7 @@ class Definitions {
     @tu lazy val Int_== : Symbol = IntClass.requiredMethod(nme.EQ, List(IntType))
     @tu lazy val Int_>= : Symbol = IntClass.requiredMethod(nme.GE, List(IntType))
     @tu lazy val Int_<= : Symbol = IntClass.requiredMethod(nme.LE, List(IntType))
+    @tu lazy val Int_>  : Symbol = IntClass.requiredMethod(nme.GT, List(IntType))
   @tu lazy val LongType: TypeRef = valueTypeRef("scala.Long", java.lang.Long.TYPE, LongEnc, nme.specializedTypeNames.Long)
   def LongClass(using Context): ClassSymbol = LongType.symbol.asClass
     @tu lazy val Long_+ : Symbol = LongClass.requiredMethod(nme.PLUS, List(LongType))

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

@@ -425,6 +425,7 @@ object StdNames {
     val array_length : N        = "array_length"
     val array_update : N        = "array_update"
     val arraycopy: N            = "arraycopy"
+    val arity: N                = "arity"
     val as: N                   = "as"
     val asTerm: N               = "asTerm"
     val asModule: N             = "asModule"

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

@@ -62,7 +62,7 @@ class FirstTransform extends MiniPhase with SymTransformer { thisPhase =>
       case Select(qual, name) if !name.is(OuterSelectName) && tree.symbol.exists =>
         val qualTpe = qual.tpe
         assert(
-          qualTpe.isErasedValueType || qualTpe.derivesFrom(tree.symbol.owner) ||
+          qualTpe.widenDealias.isErasedValueType || qualTpe.derivesFrom(tree.symbol.owner) ||
             tree.symbol.is(JavaStatic) && qualTpe.derivesFrom(tree.symbol.enclosingClass),
           i"non member selection of ${tree.symbol.showLocated} from ${qualTpe} in $tree")
       case _: TypeTree =>

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

@@ -504,53 +504,103 @@ class SyntheticMembers(thisPhase: DenotTransformer) {
   /** The class
    *
    *  ```
-   *  case class C[T <: U](x: T, y: String*)
+   *  trait U:
+   *    type Elem
+   *
+   *  case class C[T <: U](a: T, b: a.Elem, c: String*)
    *  ```
    *
    *  gets the `fromProduct` method:
    *
    *  ```
    *  def fromProduct(x$0: Product): MirroredMonoType =
-   *    new C[U](
-   *      x$0.productElement(0).asInstanceOf[U],
-   *      x$0.productElement(1).asInstanceOf[Seq[String]]: _*)
+   *    val a$1 = x$0.productElement(0).asInstanceOf[U]
+   *    val b$1 = x$0.productElement(1).asInstanceOf[a$1.Elem]
+   *    val c$1 = x$0.productElement(2).asInstanceOf[Seq[String]]
+   *    new C[U](a$1, b$1, c$1*)
    *  ```
    *  where
    *  ```
    *  type MirroredMonoType = C[?]
    *  ```
+   *
+   *  However, if the last parameter is annotated `@unroll` then we generate:
+   *
+   *  def fromProduct(x$0: Product): MirroredMonoType =
+   *    val arity = x$0.productArity
+   *    val a$1 = x$0.productElement(0).asInstanceOf[U]
+   *    val b$1 = x$0.productElement(1).asInstanceOf[a$1.Elem]
+   *    val c$1 = (
+   *      if arity > 2 then
+   *        x$0.productElement(2)
+   *      else
+   *        <default getter for the third parameter of C>
+   *    ).asInstanceOf[Seq[String]]
+   *    new C[U](a$1, b$1, c$1*)
    */
-  def fromProductBody(caseClass: Symbol, param: Tree, optInfo: Option[MirrorImpl.OfProduct])(using Context): Tree =
-    def extractParams(tpe: Type): List[Type] =
-      tpe.asInstanceOf[MethodType].paramInfos
-
-    def computeFromCaseClass: (Type, List[Type]) =
-      val (baseRef, baseInfo) =
-        val rawRef = caseClass.typeRef
-        val rawInfo = caseClass.primaryConstructor.info
-        optInfo match
-          case Some(info) =>
-            (rawRef.asSeenFrom(info.pre, caseClass.owner), rawInfo.asSeenFrom(info.pre, caseClass.owner))
-          case _ =>
-            (rawRef, rawInfo)
-      baseInfo match
+  def fromProductBody(caseClass: Symbol, productParam: Tree, optInfo: Option[MirrorImpl.OfProduct])(using Context): Tree =
+    val classRef = optInfo match
+      case Some(info) => TypeRef(info.pre, caseClass)
+      case _ => caseClass.typeRef
+    val (newPrefix, constrMeth, constrSyms) =
+      val constr = TermRef(classRef, caseClass.primaryConstructor)
+      val symss = caseClass.primaryConstructor.paramSymss
+      (constr.info: @unchecked) match
         case tl: PolyType =>
           val tvars = constrained(tl)
           val targs = for tvar <- tvars yield
             tvar.instantiate(fromBelow = false)
-          (baseRef.appliedTo(targs), extractParams(tl.instantiate(targs)))
-        case methTpe =>
-          (baseRef, extractParams(methTpe))
-    end computeFromCaseClass
-
-    val (classRefApplied, paramInfos) = computeFromCaseClass
-    val elems =
-      for ((formal, idx) <- paramInfos.zipWithIndex) yield
-        val elem =
-          param.select(defn.Product_productElement).appliedTo(Literal(Constant(idx)))
-            .ensureConforms(formal.translateFromRepeated(toArray = false))
-        if (formal.isRepeatedParam) ctx.typer.seqToRepeated(elem) else elem
-    New(classRefApplied, elems)
+          (AppliedType(classRef, targs), tl.instantiate(targs).asInstanceOf[MethodType], symss(1))
+        case mt: MethodType =>
+          (classRef, mt, symss.head)
+
+    // Index of the first parameter marked `@unroll` or -1
+    val unrolledFrom =
+      constrSyms.indexWhere(_.hasAnnotation(defn.UnrollAnnot))
+
+    // `val arity = x$0.productArity`
+    val arityDef: Option[ValDef] =
+      if unrolledFrom != -1 then
+        Some(SyntheticValDef(nme.arity, productParam.select(defn.Product_productArity).withSpan(ctx.owner.span.focus)))
+      else None
+    val arityRefTree = arityDef.map(vd => ref(vd.symbol))
+
+    // Create symbols for the vals corresponding to each parameter
+    // If there are dependent parameters, the infos won't be correct yet.
+    val bindingSyms = constrMeth.paramRefs.map: pref =>
+      newSymbol(ctx.owner, pref.paramName.freshened, Synthetic,
+        pref.underlying.translateFromRepeated(toArray = false), coord = ctx.owner.span.focus)
+    val bindingRefs = bindingSyms.map(TermRef(NoPrefix, _))
+    // Fix the infos for dependent parameters
+    if constrMeth.isParamDependent then
+      bindingSyms.foreach: bindingSym =>
+        bindingSym.info = bindingSym.info.substParams(constrMeth, bindingRefs)
+
+    def defaultGetterAtIndex(idx: Int): Tree =
+      val defaultGetterPrefix = caseClass.primaryConstructor.name.toTermName
+      ref(caseClass.companionModule).select(NameKinds.DefaultGetterName(defaultGetterPrefix, idx))
+
+    val bindingDefs = bindingSyms.zipWithIndex.map: (bindingSym, idx) =>
+      val selection = productParam.select(defn.Product_productElement).appliedTo(Literal(Constant(idx)))
+      val rhs = (
+        if unrolledFrom != -1 && idx >= unrolledFrom then
+          If(arityRefTree.get.select(defn.Int_>).appliedTo(Literal(Constant(idx))),
+            thenp =
+              selection,
+            elsep =
+              defaultGetterAtIndex(idx))
+        else
+          selection
+      ).ensureConforms(bindingSym.info)
+      ValDef(bindingSym, rhs)
+
+    val newArgs = bindingRefs.lazyZip(constrMeth.paramInfos).map: (bindingRef, paramInfo) =>
+      val refTree = ref(bindingRef)
+      if paramInfo.isRepeatedParam then ctx.typer.seqToRepeated(refTree) else refTree
+    Block(
+      arityDef.toList ::: bindingDefs,
+      New(newPrefix, newArgs)
+    )
   end fromProductBody
 
   /** For an enum T:

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

@@ -228,46 +228,9 @@ class UnrollDefinitions extends MacroTransform, IdentityDenotTransformer {
     forwarderDef
   }
 
-  private def generateFromProduct(startParamIndices: List[Int], paramCount: Int, defdef: DefDef)(using Context) = {
-    cpy.DefDef(defdef)(
-      name = defdef.name,
-      paramss = defdef.paramss,
-      tpt = defdef.tpt,
-      rhs = Match(
-        ref(defdef.paramss.head.head.asInstanceOf[ValDef].symbol).select(termName("productArity")),
-        startParamIndices.map { paramIndex =>
-          val Apply(select, args) = defdef.rhs: @unchecked
-          CaseDef(
-            Literal(Constant(paramIndex)),
-            EmptyTree,
-            Apply(
-              select,
-              args.take(paramIndex) ++
-                Range(paramIndex, paramCount).map(n =>
-                  ref(defdef.symbol.owner.companionModule)
-                    .select(DefaultGetterName(defdef.symbol.owner.primaryConstructor.name.toTermName, n))
-                )
-            )
-          )
-        } :+ CaseDef(
-          Underscore(defn.IntType),
-          EmptyTree,
-          defdef.rhs
-        )
-      )
-    ).setDefTree
-  }
-
-  private enum Gen:
-    case Substitute(origin: Symbol, newDef: DefDef)
-    case Forwarders(origin: Symbol, forwarders: List[DefDef])
+  case class Forwarders(origin: Symbol, forwarders: List[DefDef])
 
-    def origin: Symbol
-    def extras: List[DefDef] = this match
-      case Substitute(_, d) => d :: Nil
-      case Forwarders(_, ds) => ds
-
-  private def generateSyntheticDefs(tree: Tree, compute: ComputeIndices)(using Context): Option[Gen] = tree match {
+  private def generateSyntheticDefs(tree: Tree, compute: ComputeIndices)(using Context): Option[Forwarders] = tree match {
     case defdef: DefDef if defdef.paramss.nonEmpty =>
       import dotty.tools.dotc.core.NameOps.isConstructorName
 
@@ -277,38 +240,29 @@ class UnrollDefinitions extends MacroTransform, IdentityDenotTransformer {
       val isCaseApply =
         defdef.name == nme.apply && defdef.symbol.owner.companionClass.is(CaseClass)
 
-      val isCaseFromProduct = defdef.name == nme.fromProduct && defdef.symbol.owner.companionClass.is(CaseClass)
-
       val annotated =
         if (isCaseCopy) defdef.symbol.owner.primaryConstructor
         else if (isCaseApply) defdef.symbol.owner.companionClass.primaryConstructor
-        else if (isCaseFromProduct) defdef.symbol.owner.companionClass.primaryConstructor
         else defdef.symbol
 
       compute(annotated) match {
         case Nil => None
         case (paramClauseIndex, annotationIndices) :: Nil =>
           val paramCount = annotated.paramSymss(paramClauseIndex).size
-          if isCaseFromProduct then
-            Some(Gen.Substitute(
-              origin = defdef.symbol,
-              newDef = generateFromProduct(annotationIndices, paramCount, defdef)
-            ))
-          else
-            val generatedDefs =
-              val indices = (annotationIndices :+ paramCount).sliding(2).toList.reverse
-              indices.foldLeft(List.empty[DefDef]):
-                case (defdefs, paramIndex :: nextParamIndex :: Nil) =>
-                  generateSingleForwarder(
-                    defdef,
-                    paramIndex,
-                    paramCount,
-                    nextParamIndex,
-                    paramClauseIndex,
-                    isCaseApply
-                  ) :: defdefs
-                case _ => unreachable("sliding with at least 2 elements")
-            Some(Gen.Forwarders(origin = defdef.symbol, forwarders = generatedDefs))
+          val generatedDefs =
+            val indices = (annotationIndices :+ paramCount).sliding(2).toList.reverse
+            indices.foldLeft(List.empty[DefDef]):
+              case (defdefs, paramIndex :: nextParamIndex :: Nil) =>
+                generateSingleForwarder(
+                  defdef,
+                  paramIndex,
+                  paramCount,
+                  nextParamIndex,
+                  paramClauseIndex,
+                  isCaseApply
+                ) :: defdefs
+              case _ => unreachable("sliding with at least 2 elements")
+          Some(Forwarders(origin = defdef.symbol, forwarders = generatedDefs))
 
         case multiple =>
           report.error("Cannot have multiple parameter lists containing `@unroll` annotation", defdef.srcPos)
@@ -323,14 +277,12 @@ class UnrollDefinitions extends MacroTransform, IdentityDenotTransformer {
     val generatedBody = tmpl.body.flatMap(generateSyntheticDefs(_, compute))
     val generatedConstr0 = generateSyntheticDefs(tmpl.constr, compute)
     val allGenerated = generatedBody ++ generatedConstr0
-    val bodySubs = generatedBody.collect({ case s: Gen.Substitute => s.origin }).toSet
-    val otherDecls = tmpl.body.filterNot(d => d.symbol.exists && bodySubs(d.symbol))
 
     if allGenerated.nonEmpty then
-      val byName = (tmpl.constr :: otherDecls).groupMap(_.symbol.name.toString)(_.symbol)
+      val byName = (tmpl.constr :: tmpl.body).groupMap(_.symbol.name.toString)(_.symbol)
       for
         syntheticDefs <- allGenerated
-        dcl <- syntheticDefs.extras
+        dcl <- syntheticDefs.forwarders
       do
         val replaced = dcl.symbol
         byName.get(dcl.name.toString).foreach { syms =>
@@ -348,7 +300,7 @@ class UnrollDefinitions extends MacroTransform, IdentityDenotTransformer {
       tmpl.parents,
       tmpl.derived,
       tmpl.self,
-      otherDecls ++ allGenerated.flatMap(_.extras)
+      tmpl.body ++ allGenerated.flatMap(_.forwarders)
     )
   }
 

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

@@ -1957,9 +1957,7 @@ class Namer { typer: Typer =>
     if isConstructor then
       // set result type tree to unit, but take the current class as result type of the symbol
       typedAheadType(ddef.tpt, defn.UnitType)
-      val mt = wrapMethType(effectiveResultType(sym, paramSymss))
-      if sym.isPrimaryConstructor then checkCaseClassParamDependencies(mt, sym.owner)
-      mt
+      wrapMethType(effectiveResultType(sym, paramSymss))
     else
       val paramFn = if Feature.enabled(Feature.modularity) && sym.isAllOf(Given | Method) then wrapRefinedMethType else wrapMethType
       valOrDefDefSig(ddef, sym, paramSymss, paramFn)
@@ -2001,16 +1999,6 @@ class Namer { typer: Typer =>
     ddef.trailingParamss.foreach(completeParams)
   end completeTrailingParamss
 
-  /** Checks an implementation restriction on case classes. */
-  def checkCaseClassParamDependencies(mt: Type, cls: Symbol)(using Context): Unit =
-    mt.stripPoly match
-      case mt: MethodType if cls.is(Case) && mt.isParamDependent =>
-        // See issue #8073 for background
-        report.error(
-            em"""Implementation restriction: case classes cannot have dependencies between parameters""",
-            cls.srcPos)
-      case _ =>
-
   private def setParamTrackedWithAccessors(psym: Symbol, ownerTpe: Type)(using Context): Unit =
     for acc <- ownerTpe.decls.lookupAll(psym.name) if acc.is(ParamAccessor) do
       acc.resetFlag(PrivateLocal)

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

@@ -3198,7 +3198,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         .withType(dummy.termRef)
       if (!cls.isOneOf(AbstractOrTrait) && !ctx.isAfterTyper)
         checkRealizableBounds(cls, cdef.sourcePos.withSpan(cdef.nameSpan))
-      if cls.isEnum || firstParentTpe.classSymbol.isEnum then
+      if cls.isEnum || !cls.isRefinementClass && firstParentTpe.classSymbol.isEnum then
         checkEnum(cdef, cls, firstParent)
       val cdef1 = assignType(cpy.TypeDef(cdef)(name, impl1), cls)
 

tests/pos/enum-refinement.scala

@@ -0,0 +1,12 @@
+enum Enum:
+  case EC(val x: Int)
+
+val a: Enum.EC { val x: 1 } = Enum.EC(1).asInstanceOf[Enum.EC { val x: 1 }]
+
+import scala.language.experimental.modularity
+
+enum EnumT:
+  case EC(tracked val x: Int)
+
+val b: EnumT.EC { val x: 1 } = EnumT.EC(1)
+