Fix scala#10123: Try to fully define qualifier type

Try to fully define qualifier type before emitting an error for
a typed Select.

The one possible objection to doing this is that it might hide other
errors where a type was not sufficiently well defined before we
select on it. On the other hand:

 - My comment in issue scala#10123 shows that the problem is more widespread
   than just a single case.
 - We otherwise rely on interpolation to instantiate type variables in
   many cases. But interpolation is meant as an optimization only, to
   keep constraint sets smaller. So, we should be prepared that interpolation
   will not do its job. The fix in this commit is a way to do so.

Author: odersky

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

@@ -12,6 +12,7 @@ import config.Printers.typr
 import ast.Trees._
 import NameOps._
 import ProtoTypes._
+import Inferencing.isFullyDefined
 import collection.mutable
 import reporting._
 import Checking.{checkNoPrivateLeaks, checkNoWildcard}
@@ -159,6 +160,9 @@ trait TypeAssigner {
       TryDynamicCallType
     else if (qualType.isErroneous || name.toTermName == nme.ERROR)
       UnspecifiedErrorType
+    else if !isFullyDefined(qualType, ForceDegree.none) && isFullyDefined(qualType, ForceDegree.failBottom) then
+      // try again with more defined qualifier type
+      selectionType(tree, qual1)
     else if (name == nme.CONSTRUCTOR)
       errorType(ex"$qualType does not have a constructor", tree.srcPos)
     else {

tests/pos/i10123.scala

@@ -0,0 +1,19 @@
+class D
+class C[+T](x: T)
+
+class Foo() {
+  val status: Int = 0
+}
+
+object Main {
+  implicit class RichC[T](c: C[T]) {
+    def await(implicit d: D = ???): T = ???
+  }
+
+  def test1: Int = {
+    val foo = new C(new Foo()).await
+    foo.status
+  }
+
+  val test2 = new C(new Foo()).await.status
+}