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.

src/compiler/scala/tools/nsc/transform/ExplicitOuter.scala

@@ -112,6 +112,29 @@ abstract class ExplicitOuter extends InfoTransform
     sym setInfo clazz.outerClass.thisType
   }
 
+  /**
+   * Will the outer accessor of the `clazz` subsume the outer accessor of
+   * `mixin`?
+   *
+   * This arises when an inner object mixes in its companion trait.
+   *
+   * {{{
+   *   class C {
+   *     trait T { C.this }            // C$T$$$outer$ : C
+   *     object T extends T { C.this } // C$T$$$outer$ : C.this.type
+   *   }
+   * }}}
+   *
+   * See SI-7242.
+   }}
+   */
+  private def skipMixinOuterAccessor(clazz: Symbol, mixin: Symbol) = {
+    // Reliant on the current scheme for name expansion, the expanded name
+    // of the outer accessors in a trait and its companion object are the same.
+    // If the assumption is one day falsified, run/t7424.scala will let us know.
+    clazz.fullName == mixin.fullName
+  }
+
   /** <p>
    *    The type transformation method:
    *  </p>
@@ -177,10 +200,14 @@ abstract class ExplicitOuter extends InfoTransform
         for (mc <- clazz.mixinClasses) {
           val mixinOuterAcc: Symbol = afterExplicitOuter(outerAccessor(mc))
           if (mixinOuterAcc != NoSymbol) {
-            if (decls1 eq decls) decls1 = decls.cloneScope
-            val newAcc = mixinOuterAcc.cloneSymbol(clazz, mixinOuterAcc.flags & ~DEFERRED)
-            newAcc setInfo (clazz.thisType memberType mixinOuterAcc)
-            decls1 enter newAcc
+            if (skipMixinOuterAccessor(clazz, mc))
+              debuglog(s"Reusing outer accessor symbol of $clazz for the mixin outer accessor of $mc")
+            else {
+              if (decls1 eq decls) decls1 = decls.cloneScope
+              val newAcc = mixinOuterAcc.cloneSymbol(clazz, mixinOuterAcc.flags & ~DEFERRED)
+              newAcc setInfo (clazz.thisType memberType mixinOuterAcc)
+              decls1 enter newAcc
+            }
           }
         }
       }
@@ -390,6 +417,7 @@ abstract class ExplicitOuter extends InfoTransform
       val outerAcc    = outerAccessor(mixinClass) overridingSymbol currentClass
       def mixinPrefix = (currentClass.thisType baseType mixinClass).prefix
       assert(outerAcc != NoSymbol, "No outer accessor for inner mixin " + mixinClass + " in " + currentClass)
+      assert(outerAcc.alternatives.size == 1, s"Multiple outer accessors match inner mixin $mixinClass in $currentClass : ${outerAcc.alternatives.map(_.defString)}")
       // I added the mixinPrefix.typeArgs.nonEmpty condition to address the
       // crash in SI-4970.  I feel quite sure this can be improved.
       val path = (
@@ -492,7 +520,7 @@ abstract class ExplicitOuter extends InfoTransform
               }
               if (!currentClass.isTrait)
                 for (mc <- currentClass.mixinClasses)
-                  if (outerAccessor(mc) != NoSymbol)
+                  if (outerAccessor(mc) != NoSymbol && !skipMixinOuterAccessor(currentClass, mc))
                     newDefs += mixinOuterAccessorDef(mc)
             }
           }

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
+}

test/files/run/t7242.scala

@@ -0,0 +1,71 @@
+class CrashTest {
+  def foo = ()
+  trait CrashTestTable {
+    def cols = foo
+  }
+  // This was leading to a class between the mixed in
+  // outer accessor and the outer accessor of this object.
+  object CrashTestTable extends CrashTestTable {
+    foo
+    cols
+  }
+}
+
+class CrashTest1 {
+  def foo = ()
+  class CrashTestTable {
+    def cols = foo
+  }
+  object CrashTestTable extends CrashTestTable {
+    foo
+    cols
+  }
+}
+
+class CrashTest2 {
+  def foo = ()
+  trait CrashTestTable {
+    def cols = foo
+  }
+  object Obj extends CrashTestTable {
+    foo
+    cols
+  }
+}
+
+class CrashTest3 {
+  def foo = ()
+
+  def meth() {
+    trait CrashTestTable {
+      def cols = foo
+    }
+    object Obj extends CrashTestTable {
+      foo
+      cols
+    }
+    Obj
+  }
+}
+
+object Test extends App {
+  {
+    val c = new CrashTest
+    c.CrashTestTable
+  }
+
+  {
+    val c = new CrashTest1
+    c.CrashTestTable
+  }
+
+  {
+    val c = new CrashTest2
+    c.Obj
+  }
+
+  {
+    val c = new CrashTest3
+    c.meth()
+  }
+}