[Draft] A more dependent pattern matching prototype

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

@@ -810,8 +810,12 @@ object desugar {
           for i <- List.range(0, arity)
               selName = nme.selectorName(i)
               if (selName ne caseParams(i).name) && !selectorNamesInBody.contains(selName)
-          yield syntheticProperty(selName, caseParams(i).tpt,
-            Select(This(EmptyTypeIdent), caseParams(i).name))
+          yield
+            val ptp =
+              if caseParams(i).mods.mods.exists(_.isInstanceOf[Mod.Var]) then caseParams(i).tpt
+              else SingletonTypeTree(Select(This(EmptyTypeIdent), caseParams(i).name))
+            syntheticProperty(selName, ptp,
+              Select(This(EmptyTypeIdent), caseParams(i).name))
 
       def enumCaseMeths =
         if isEnumCase then
@@ -918,7 +922,10 @@ object desugar {
             if (arity == 0) Literal(Constant(true))
             else if caseClassInScala2Library then scala2LibCompatUnapplyRhs(unapplyParam.name)
             else Ident(unapplyParam.name)
-          val unapplyResTp = if (arity == 0) Literal(Constant(true)) else TypeTree()
+          val unapplyResTp =
+            if (arity == 0) Literal(Constant(true))
+            else if caseClassInScala2Library then TypeTree()
+            else SingletonTypeTree(Ident(unapplyParam.name))
 
           DefDef(
             methName,

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

@@ -1625,7 +1625,8 @@ trait Applications extends Compatibility {
             val patternBound = maximizeType(unapplyArgType, unapplyFn.span.endPos)
             if (patternBound.nonEmpty) unapplyFn = addBinders(unapplyFn, patternBound)
             unapp.println(i"case 2 $unapplyArgType ${ctx.typerState.constraint}")
-            unapplyArgType
+            selType & unapplyArgType
+            // unapplyArgType
 
         val dummyArg = dummyTreeOfType(ownType)
         val (newUnapplyFn, unapplyApp) =
@@ -1637,7 +1638,7 @@ trait Applications extends Compatibility {
           .typedPatterns(qual, this)
         val result = assignType(cpy.UnApply(tree)(newUnapplyFn, unapplyImplicits(dummyArg, unapplyApp), unapplyPatterns), ownType)
         if (ownType.stripped eq selType.stripped) || ownType.isError then result
-        else tryWithTypeTest(Typed(result, TypeTree(ownType)), selType)
+        else tryWithTypeTest(Typed(result, TypeTree(unapplyArgType)), selType)
       case tp =>
         val unapplyErr = if (tp.isError) unapplyFn else notAnExtractor(unapplyFn)
         val typedArgsErr = unadaptedArgs.mapconserve(typed(_, defn.AnyType))

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

@@ -593,7 +593,10 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     typr.println(s"typed ident $kind$name in ${ctx.owner}")
     if ctx.mode.is(Mode.Pattern) then
       if name == nme.WILDCARD then
-        return tree.withType(pt)
+        val wpt = pt match
+          case pt: TermRef => pt.widen
+          case pt => pt
+        return tree.withType(wpt)
       if name == tpnme.WILDCARD then
         return tree.withType(defn.AnyType)
       if untpd.isVarPattern(tree) && name.isTermName then
@@ -2031,7 +2034,12 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         val rawSelectorTpe = fullyDefinedType(sel1.tpe, "pattern selector", tree.srcPos)
         val selType = rawSelectorTpe match
           case c: ConstantType if tree.isInline => c
+          case ref: TermRef => ref
           case otherTpe => otherTpe.widen
+        val selTypeW = selType match
+          case tr: TermRef => tr.widen
+          case _ => selType
+
 
         /** Does `tree` has the same shape as the given match type?
          *  We only support typed patterns with empty guards, but
@@ -2049,7 +2057,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                 // To check that pattern types correspond we need to type
                 // check `pat` here and throw away the result.
                 val gadtCtx: Context = ctx.fresh.setFreshGADTBounds
-                val pat1 = typedPattern(pat, selType)(using gadtCtx)
+                val pat1 = typedPattern(pat, selTypeW)(using gadtCtx)
                 val tpt = tpd.unbind(tpd.unsplice(pat1)) match
                   case Typed(_, tpt) => tpt
                   case UnApply(fun, _, p1 :: _) if fun.symbol == defn.TypeTest_unapply => p1
@@ -2067,7 +2075,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
 
         val result = pt.underlyingNormalizable match {
           case mt: MatchType if isMatchTypeShaped(mt) =>
-            typedDependentMatchFinish(tree, sel1, selType, tree.cases, mt)
+            typedDependentMatchFinish(tree, sel1, selTypeW, tree.cases, mt)
           case _ =>
             typedMatchFinish(tree, sel1, selType, tree.cases, pt)
         }
@@ -2677,6 +2685,9 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
   def typedBind(tree: untpd.Bind, pt: Type)(using Context): Tree = {
     if !isFullyDefined(pt, ForceDegree.all) then
       return errorTree(tree, em"expected type of $tree is not fully defined")
+    val wpt = pt match
+      case pt: TermRef => pt.widen
+      case _ => pt
     val body1 = typed(tree.body, pt)
     body1 match {
       case UnApply(fn, Nil, arg :: Nil)
@@ -2705,9 +2716,9 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           val symTp =
             if isStableIdentifierOrLiteral || pt.isNamedTupleType then pt
               // need to combine tuple element types with expected named type
-            else if isWildcardStarArg(body1)
-                    || pt == defn.ImplicitScrutineeTypeRef
-                    || body1.tpe <:< pt  // There is some strange interaction with gadt matching.
+            else if isWildcardStarArg(body1) || pt == defn.ImplicitScrutineeTypeRef
+            then body1.tpe
+            else if body1.tpe <:< wpt  // There is some strange interaction with gadt matching.
                                          // and implicit scopes.
                                          // run/t2755.scala fails to compile if this subtype test is omitted
                                          // and the else clause is changed to `body1.tpe & pt`. What
@@ -2717,14 +2728,18 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                                          // it is Array[T] we get an implicit not found. To avoid fragility
                                          // wrt to operand order for `&`, we include the explicit subtype test here.
                                          // See also #5649.
-            then body1.tpe
+            then
+              if ctx.mode.is(Mode.Pattern) && (pt ne wpt) // && !(body1.tpe <:< pt)
+              then AndType(pt, body1.tpe)
+              // then pt & body1.tpe
+              else body1.tpe
             else body1.tpe match
               case btpe: TypeRef
               if btpe.symbol == defn.TupleXXLClass && pt.tupleElementTypes.isDefined =>
                 // leave the original tuple type; don't mix with & TupleXXL which would only obscure things
                 pt
               case _ =>
-                pt & body1.tpe
+                wpt & body1.tpe
           val sym = newPatternBoundSymbol(name, symTp, tree.span)
           if (pt == defn.ImplicitScrutineeTypeRef || tree.mods.is(Given)) sym.setFlag(Given)
           if (ctx.mode.is(Mode.InPatternAlternative))

library/src/scala/Tuple.scala

@@ -265,7 +265,7 @@ object Tuple {
 
   /** Convert an array into a tuple of unknown arity and types */
   def fromArray[T](xs: Array[T]): Tuple = {
-    val xs2 = xs match {
+    val xs2: Array[Object] = xs match {
       case xs: Array[Object] => xs
       case xs => xs.map(_.asInstanceOf[Object])
     }

tests/pos/dependent-pattern-match-type-test.scala

@@ -0,0 +1,17 @@
+//> using options -Xfatal-warnings
+
+import scala.reflect.TypeTest
+
+trait M:
+  type Tree <: AnyRef
+  type Apply <: Tree
+  given TypeTest[Tree, Apply] = ???
+  val Apply: ApplyModule
+  trait ApplyModule:
+    this: Apply.type =>
+    def unapply(x: Apply): (Tree, Tree) = ???
+
+  def quote(x: Tree) = x match
+    case Apply(f, args) =>
+      println(args)
+    case _ =>

tests/pos/dependent-pattern-match.scala

@@ -0,0 +1,59 @@
+trait A:
+  val x: Int
+  type T
+case class B(x: Int, y: Int) extends A:
+  type T = String
+  val z: T = "B"
+case class C(x: Int) extends A:
+  type T = Int
+  val z: T = 1
+
+object BParts:
+  def unapply(b: B): Option[(b.x.type, b.y.type)] = Some((b.x, b.y))
+
+def test1(a: A) = a match
+  case b @ B(x, y) =>
+    // b: a.type & B
+    // x: (a.type & B).x.type
+    // y: (a.type & B).y.type
+    val e1: a.type = b
+    val e2: a.x.type = b.x
+    val e3: a.x.type = x
+    val e4: b.x.type = x
+    x + y
+  case C(x) =>
+    val e1: a.x.type = x
+    x
+  case BParts(x, y) =>
+    val e1: a.x.type = x
+    x + y
+  case _ => 0
+
+def test2(a: A): a.T =
+  a match
+    case b: B =>
+      // b: a.type & B
+      // b.z: b.T = (a & B)#T = a.T & String
+      b.z
+    case c: C => c.z
+
+def test3(a: A): a.T =
+  a match
+    case b: B =>
+      // b: a.type & B; hence b.T <:< a.T & String
+      // We don't have a: b.type in the body,
+      // so we can't prove String <:< a.T
+      val x: b.T = b.z + ""
+      x
+    case c: C =>
+      val x: c.T = c.z + 0
+      x
+
+def test4(x: A, y: A) =
+  x match
+    case z: y.type =>
+      // if x.eq(y) then z = x = y,
+      // z: x.type & y.type
+      val a: x.type = z
+      val b: y.type = z
+    case _ =>