Better error message when unapply method is missing

Author: odersky

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

@@ -814,9 +814,11 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
     def followTypeAlias(tree: untpd.Tree): untpd.Tree = {
       tree match {
         case tree: untpd.RefTree =>
-          val ttree = typedType(untpd.rename(tree, tree.name.toTypeName))
+          val nestedCtx = ctx.fresh.setNewTyperState
+          val ttree =
+            typedType(untpd.rename(tree, tree.name.toTypeName))(nestedCtx)
           ttree.tpe match {
-            case alias: TypeRef if alias.info.isAlias =>
+            case alias: TypeRef if alias.info.isAlias && !nestedCtx.reporter.hasErrors =>
               companionRef(alias) match {
                 case companion: TermRef => return untpd.ref(companion) withPos tree.pos
                 case _ =>

tests/neg/i2378.scala

@@ -0,0 +1,30 @@
+trait Cap
+
+trait Toolbox {
+  type Tree
+
+  val tpd: TypedTrees
+  trait TypedTrees {
+    type Tree
+  }
+
+  val Apply: ApplyImpl
+
+  trait ApplyImpl {
+    def unapply(tree: Tree): Option[(Tree, Seq[Tree])]
+    def unapply(tree: tpd.Tree)(implicit c: Cap): Option[(tpd.Tree, Seq[tpd.Tree])]
+  }
+}
+
+class Test(val tb: Toolbox) {
+  import tb._
+  implicit val cap: Cap = null
+
+  def foo(tree: Tree): Int = tree match { // error: private escape
+    case tb.Apply(fun, args) => 3 // error
+  }
+
+  def bar(tree: tpd.Tree): Int = tree match {  // error: private escape
+    case Apply(fun, args) => 3 // error
+  }
+}