SI-7801 Fix a nightmarish bug in Symbols#adaptInfos

The compiler-in-residence has always been a sketchy affair;
FSC and REPL offers a bounty of bugs that exploit the menagerie
of time-travel mechanisms in play for symbols' metadata (type, flags,
name and owner.) but are often cleverly masked by optimizations in
the compiler based on reference equality.

The latest: an innocuous change in Erasure:

  https://github.com/scala/scala/commit/d8b96bb8#commitcomment-3995163

means that some `ErasureMap`-s over `MethodType`-s are now true
identities (as `UnitTpe` is always the same object, whereas
`erasedTypeRef(UnitClass)` returns an different `TypeRef` each
time.)

This, in turn, enables `TypeMap#mapOver` to reuse
the existing enclosing type, and so on. On such subtleties hinge
further optimizations, such as whether or not a given phase's
`InfoTransformer` needs to add an entry in a symbols type history.

When the REPL (or FSC / Presentation Compiler) creates a new
`Run`, `Symbol#rawInfo` tries to adapt the entries in the type
history for the new run. For packages, this was taken to be a
no-op; each entry is marked as being valid in the new run and
no further action is taken. This logic lurks in `adaptInfos`.

But, when the namer enters a new symbol in a package, it
*mutates* the Scope of that package classes info `enteringTyper`.
So the later entries in the type history *must* be invalidated
and recomputed.

We have two choices for a fix:

 1) modify `Namers#enterInScope` to blow away the subsequent
    type history for the owning symbol after inserting the
    new member. Something like `owner.setInfo(owner.info)` would
    have the desired effect.
 2) Change `adaptInfos` to be more conservative when it comes
    to package classes, and retain only the oldest entry in the
    type history.

This commit goes for option 2.

2 files changed, 23 insertions, 0 deletions

diff --git a/test/files/run/t7801.check b/test/files/run/t7801.check
new file mode 100644
index 0000000000..d72060c684
--- /dev/null
+++ b/test/files/run/t7801.check
@@ -0,0 +1,11 @@
+Type in expressions to have them evaluated.
+Type :help for more information.
+
+scala> val g: scala.reflect.internal.SymbolTable = null; import g.abort
+g: scala.reflect.internal.SymbolTable = null
+import g.abort
+
+scala> class C(val a: Any) extends AnyVal
+defined class C
+
+scala> 
diff --git a/test/files/run/t7801.scala b/test/files/run/t7801.scala
new file mode 100644
index 0000000000..3a3cc97a51
--- /dev/null
+++ b/test/files/run/t7801.scala
@@ -0,0 +1,12 @@
+import scala.tools.partest.ReplTest
+
+// was crashing due to a subtle interaction of the Namer entering packages into
+// enclosing packages by mutating the scope in place without invalidating later
+// entries in the enclosing package class symbols type history.
+//
+// Sadly, I couldn't whittle the test case down further.
+object Test extends ReplTest {
+  override def code = """val g: scala.reflect.internal.SymbolTable = null; import g.abort
+                        |class C(val a: Any) extends AnyVal""".stripMargin
+
+}