Fixes to double binding check.

There was a spurious error when doing pickling tests with -YnoDoubleBindings
The test manifested itself when compiling `Trees` and `untpd`. The problem was
a sourious double assignment to the `untpd$` TypeRef. The problem was that
the old `untpd` symbol did get a runId of 3, because it was looked at immediately
before being overritten with the new symbol. So the system detected a race condition.

The new test looks at the denotations previous run id instead of the symbol's when
setting a denotation. That runId was still 2, so not real double binding occurred.

1 files changed, 8 insertions, 3 deletions

diff --git a/src/dotty/tools/dotc/core/Types.scala b/src/dotty/tools/dotc/core/Types.scala
index 9d692f39f..363e3006a 100644
--- a/src/dotty/tools/dotc/core/Types.scala
+++ b/src/dotty/tools/dotc/core/Types.scala
@@ -1256,9 +1256,14 @@ object Types {
     private def checkSymAssign(sym: Symbol)(implicit ctx: Context) =
       assert(
         (lastSymbol eq sym) ||
-        (lastSymbol eq null) ||
-        (lastSymbol.defRunId != sym.defRunId) ||
-        (lastSymbol.defRunId == NoRunId) ||
+        (lastSymbol eq null) || {
+          val lastDefRunId = lastDenotation match {
+            case d: SymDenotation => d.validFor.runId
+            case _ => lastSymbol.defRunId
+          }
+          (lastDefRunId != sym.defRunId) ||
+          (lastDefRunId == NoRunId)
+        } ||
         (lastSymbol.infoOrCompleter == ErrorType ||
         defn.overriddenBySynthetic.contains(lastSymbol)
           // for overriddenBySynthetic symbols a TermRef such as SomeCaseClass.this.hashCode