Fix #2950: prefer inner error

The inner error is supposed to be more informative thus should
be preferred.

Author: liufengyun

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

@@ -940,7 +940,9 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
   def typedUnApply(tree: untpd.Apply, selType: Type)(implicit ctx: Context): Tree = track("typedUnApply") {
     val Apply(qual, args) = tree
 
-    def notAnExtractor(tree: Tree) = errorTree(tree, NotAnExtractor(qual))
+    def notAnExtractor(tree: Tree) =
+      if (tree.tpe.isErroneous) tree
+      else errorTree(tree, NotAnExtractor(qual))
 
     /** If this is a term ref tree, try to typecheck with its type name.
      *  If this refers to a type alias, follow the alias, and if

tests/neg/i2950.scala

@@ -0,0 +1,9 @@
+class T1 {
+  case class Foo(x: Int, xs1: List[String], xs2: List[String])
+}
+
+object T2 {
+  val foo: T1#Foo = ???
+
+  val Foo(x1, xs1, xs2) = foo // error
+}