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, 8 insertions, 5 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index 70acb03584..64fbad04c0 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -5134,16 +5134,19 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
             typed(tree.ref, MonoQualifierModes | mode.onlyTypePat, AnyRefTpe)
           }
 
-        if (!refTyped.isErrorTyped)
+        if (refTyped.isErrorTyped) {
+          setError(tree)
+        } else {
           tree setType refTyped.tpe.resultType
-
-        if (treeInfo.admitsTypeSelection(refTyped)) tree
-        else UnstableTreeError(refTyped)
+          if (refTyped.isErrorTyped || treeInfo.admitsTypeSelection(refTyped)) tree
+          else UnstableTreeError(tree)
+        }
       }
 
       def typedSelectFromTypeTree(tree: SelectFromTypeTree) = {
         val qual1 = typedType(tree.qualifier, mode)
-        if (qual1.tpe.isVolatile) TypeSelectionFromVolatileTypeError(tree, qual1)
+        if (qual1.isErrorTyped) setError(treeCopy.SelectFromTypeTree(tree, qual1, tree.name))
+        else if (qual1.tpe.isVolatile) TypeSelectionFromVolatileTypeError(tree, qual1)
         else typedSelect(tree, qual1, tree.name)
       }