Fix scala#1750: Handle illegal class overrides better

Illegal class overrides are fundamentally at odds with the way dotty
represents types and therefore can cause lots of low-level problems.

Two measures in this commit

First, we detect direct illegal class overrides on completion instead of
during RefChecks. Break the override by making the previously
overriding type private.

This fixes i1750.scala, but still fails for indirect overrides between
two unrelated outer traits/classes that are inherited by the same class or trait.
We fix this by catching the previously thrown ClassCastException
in both ExtractAPI and RefChecks.

Test case for indirect overrides is in i1750a.scala.

Author: odersky

compiler/src/dotty/tools/dotc/sbt/ExtractAPI.scala

@@ -6,6 +6,7 @@ import core._, core.Decorators._
 import Annotations._, Contexts._, Flags._, Phases._, Trees._, Types._, Symbols._
 import Names._, NameOps._, StdNames._
 import typer.Inliner
+import typer.ErrorReporting.cyclicErrorMsg
 
 import dotty.tools.io.Path
 import java.io.PrintWriter
@@ -190,7 +191,16 @@ private class ExtractAPICollector(implicit val ctx: Context) extends ThunkHolder
   def apiClassStructure(csym: ClassSymbol): api.Structure = {
     val cinfo = csym.classInfo
 
-    val bases = linearizedAncestorTypes(cinfo)
+    val bases =
+      try linearizedAncestorTypes(cinfo)
+      catch {
+        case ex: CyclicReference =>
+          // See neg/i1750a for an example where a cyclic error can arise.
+          // The root cause in this example is an illegal "override" of an inner trait
+          ctx.error(cyclicErrorMsg(ex), csym.pos)
+          defn.ObjectType :: Nil
+       }
+
     val apiBases = bases.map(apiType)
 
     // Synthetic methods that are always present do not affect the API

tests/neg/overrides.scala

@@ -34,11 +34,6 @@ package p2 { // all being in the same package compiles fine
     }
   }
 
-  abstract class T3 extends T2 {
-    class A {   // error: classes cannot be overridden
-      bug()
-    }
-  }
 }
 
 class A[T] {

tests/neg/t7278.scala

@@ -1,5 +1,5 @@
 class A { class E }
-class B extends A { class E }
+class B extends A { class EB }
 trait C { type E = Int }
 trait D { type E = String }
 trait EC { type E }