Merge pull request #8816 from dotty-staging/fix-8730-v2

Fix #8730: disallow structural unapply

Author: smarter

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -1806,7 +1806,7 @@ object messages {
         |""".stripMargin
   }
 
-  class UnapplyInvalidReturnType(unapplyResult: Type, unapplyName: Symbol#ThisName)(implicit ctx: Context)
+  class UnapplyInvalidReturnType(unapplyResult: Type, unapplyName: Name)(implicit ctx: Context)
     extends DeclarationMsg(UnapplyInvalidReturnTypeID) {
     def msg =
       val addendum =

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

@@ -150,7 +150,10 @@ object Applications {
 
   def unapplyArgs(unapplyResult: Type, unapplyFn: Tree, args: List[untpd.Tree], pos: SourcePosition)(using Context): List[Type] = {
 
-    val unapplyName = unapplyFn.symbol.name
+    val unapplyName = unapplyFn match            // tolerate structural `unapply`, which does not have a symbol
+      case TypeApply(fn: RefTree, _) => fn.name
+      case fn: RefTree => fn.name
+
     def getTp = extractorMemberType(unapplyResult, nme.get, pos)
 
     def fail = {
@@ -1227,6 +1230,9 @@ trait Applications extends Compatibility {
           case Apply(Apply(unapply, `dummyArg` :: Nil), args2) => assert(args2.nonEmpty); args2
           case Apply(unapply, `dummyArg` :: Nil) => Nil
           case Inlined(u, _, _) => unapplyImplicits(u)
+          case DynamicUnapply(_) =>
+            ctx.error("Structural unapply is not supported", unapplyFn.sourcePos)
+            Nil
           case _ => Nil.assertingErrorsReported
         }
 

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

@@ -22,6 +22,17 @@ object Dynamic {
     name == nme.applyDynamic || name == nme.selectDynamic || name == nme.updateDynamic || name == nme.applyDynamicNamed
 }
 
+object DynamicUnapply {
+  def unapply(tree: tpd.Tree): Option[List[tpd.Tree]] = tree match
+    case TypeApply(Select(qual, name), _) if name == nme.asInstanceOfPM =>
+      unapply(qual)
+    case Apply(Apply(Select(selectable, fname), Literal(Constant(name)) :: ctag :: Nil), _ :: implicits)
+    if fname == nme.applyDynamic && (name == "unapply" || name == "unapplySeq") =>
+      Some(selectable :: ctag :: implicits)
+    case _ =>
+      None
+}
+
 /** Handles programmable member selections of `Dynamic` instances and values
  *  with structural types. Two functionalities:
  *

tests/neg/i8730.scala

@@ -0,0 +1,18 @@
+import reflect.Selectable.reflectiveSelectable
+
+class Nat(val x: Int) {
+  def get: Int = x
+  def isEmpty = x < 0
+}
+
+val SomeExtractorBuilder: { def unapply(x: Int): Nat } = new {
+  def unapply(x: Int): Nat = new Nat(x)
+}
+
+
+@main
+def Test = 5 match {
+  case SomeExtractorBuilder(n) => println(s"$n is a natural number") // error
+  case _      => ()
+}
+