SI-8534 Avoid crash in erroneous SelectFromTypeTree

PR #2374 changed the behaviour of `typedSingletonTypeTree` in the
presence of an error typed reference tree. It incorrectly
returns the reference tree in case on an error. However, this is
a term tree, which is an inconsistent result with the input type
tree. Consequently, a `typedSelectInternal` later fails when
using this as the qualifier of a `SelectFromTypeTree`.

Both test cases enclosed show this symptom.

This commit:

  - Returns `tree` rather than `refTyped` when `refTyped` is
    error typed or when it isn't suitable as a stable prefix.
  - Avoids issuing a cascading "not a stable prefix" error if the
    `refTyped` is error typed.
  - Adds an extra layer of defense in `typedSelectFromTypeTree`
    to bail out quickly if the qualifier is error typed.

The last measure is not necessary to fix this bug.

1 files changed, 4 insertions, 4 deletions

diff --git a/test/files/neg/t963.check b/test/files/neg/t963.check
index 4dc202c7bd..483e53c77d 100644
--- a/test/files/neg/t963.check
+++ b/test/files/neg/t963.check
@@ -1,9 +1,9 @@
-t963.scala:14: error: stable identifier required, but Test.this.y3.x found.
+t963.scala:14: error: stable identifier required, but y3.x.type found.
   val w3 : y3.x.type = y3.x
-              ^
-t963.scala:17: error: stable identifier required, but Test.this.y4.x found.
+               ^
+t963.scala:17: error: stable identifier required, but y4.x.type found.
   val w4 : y4.x.type = y4.x
-              ^
+               ^
 t963.scala:10: error: type mismatch;
  found   : AnyRef{def x: Integer}
  required: AnyRef{val x: Integer}