Set incomplete rootTreeOrProvider in typer

During typer, set `rootTreeOrProvider` to be the typed class def. This
information will be overwritten in `SetRootTree` to include the whole
package structure, but we may never reach this phase.

Author: Duhemm

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

@@ -1642,6 +1642,12 @@ class Typer extends Namer
       // check value class constraints
       checkDerivedValueClass(cls, body1)
 
+
+      // Temporarily set the typed class def as root tree so that we have at least some
+      // information in the IDE in case we never reach `SetRootTree`.
+      if (ctx.mode.is(Mode.Interactive) && ctx.settings.YretainTrees.value)
+        cls.rootTreeOrProvider = cdef1
+
       cdef1
 
       // todo later: check that