SI-7242 Fix crash when inner object mixes in its companion

Given:

    class C {
      trait T { C.this }            // C$T$$$outer$ : C
      object T extends T { C.this } // C$T$$$outer$ : C.this.type
    }

object T ended up with a definitions for both of the accessors.
These cannot co-exist, as they have the same erased type. A crash
ensued almost immediately in explitouter.

Fortunately, the solution is straightforward: we can just omit
the mixin outer accessor altogether, the objects own outer accessor
is compatible with it.

    scala> :javap C.T
    Compiled from "<console>"
    public interface C$T{
        public abstract C C$T$$$outer();
    }

    scala> :javap C.T$
    Compiled from "<console>"
    public class C$T$ extends java.lang.Object implements C$T{
        public C C$T$$$outer();
        public C$T$(C);
    }

I also added an assertion to give a better error message in
case we find ourselves here again.

Also includes tests from SI-3994, which I'll mark as a duplicate.

1 files changed, 20 insertions, 0 deletions

diff --git a/test/files/run/t3994.scala b/test/files/run/t3994.scala
new file mode 100644
index 0000000000..0ee1d9d966
--- /dev/null
+++ b/test/files/run/t3994.scala
@@ -0,0 +1,20 @@
+trait T {
+  trait Default { def foo = this }
+  object Default extends Default
+}
+
+class Crash { // if you change this to a `trait` it keeps failing, though if it is an `object` it compiles just fine!
+  class Element
+
+  /* declare this as a class, and the crash goes away */
+  trait ElementOrdering extends Ordering[Element] {
+    def compare(a: Element, b: Element): Int = 0
+  }
+
+  implicit object ElementOrdering extends ElementOrdering
+}
+
+object Test extends App {
+  (new T {}).Default
+  (new Crash).ElementOrdering
+}