Survive bad symbols of module vals

Also: fix a cyclic reference caused by 5252b15

Author: odersky

compiler/src/dotty/tools/dotc/core/SymDenotations.scala

@@ -1467,7 +1467,11 @@ object SymDenotations {
         onBehalf.signalProvisional()
       val builder = new BaseDataBuilder
       for (p <- classParents) {
-        p.underlyingClassRef(refinementOK = false).typeSymbol match {
+      	var pcls = p.typeSymbol
+      	if (!pcls.isClass) pcls = p.underlyingClassRef(refinementOK = false).typeSymbol
+      		// This roundabout way is necessary for avoiding cyclic references.
+      		// A test case is CompilationTests.compileMixed
+      	pcls match {
           case pcls: ClassSymbol => builder.addAll(pcls.baseClasses)
           case _ => assert(isRefinementClass || ctx.mode.is(Mode.Interactive), s"$this has non-class parent: $p")
         }

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

@@ -164,8 +164,11 @@ object NamerContextOps {
   /** Find moduleClass/sourceModule in effective scope */
   private def findModuleBuddy(name: Name, scope: Scope)(implicit ctx: Context) = {
     val it = scope.lookupAll(name).filter(_ is Module)
-    assert(it.hasNext, s"no companion $name in $scope")
-    it.next()
+    if (it.hasNext) it.next()
+    else {
+      assert(ctx.reporter.errorsReported, s"no companion $name in $scope")
+      NoSymbol
+    }
   }
 }
 
@@ -1033,8 +1036,11 @@ class Namer { typer: Typer =>
    */
   def moduleValSig(sym: Symbol)(implicit ctx: Context): Type = {
     val clsName = sym.name.moduleClassName
-    val cls = ctx.denotNamed(clsName) suchThat (_ is ModuleClass)
-    ctx.owner.thisType select (clsName, cls)
+    val cls = ctx.denotNamed(clsName).suchThat(_ is ModuleClass).orElse {
+      assert(ctx.reporter.errorsReported)
+      ctx.newStubSymbol(ctx.owner, clsName)
+    }
+    ctx.owner.thisType.select(clsName, cls)
   }
 
   /** The type signature of a ValDef or DefDef

tests/neg/parser-stability-14.scala

@@ -0,0 +1,4 @@
+object x0 {
+{
+val x1 x0   // error
+object x1   // error // error

tests/neg/parser-stability-16.scala

@@ -0,0 +1,5 @@
+class x0[x0] {
+  val x1 : x0
+}
+trait x3 extends x0 {
+x1 = 0 object  // error // error