Ensuring that contexts do not have NoSymbol as owner.

1 files changed, 5 insertions, 1 deletions

diff --git a/src/dotty/tools/dotc/core/Contexts.scala b/src/dotty/tools/dotc/core/Contexts.scala
index ac0e243ff..816216a98 100644
--- a/src/dotty/tools/dotc/core/Contexts.scala
+++ b/src/dotty/tools/dotc/core/Contexts.scala
@@ -276,6 +276,10 @@ object Contexts {
     final def addMode(mode: Mode): Context = withMode(this.mode | mode)
     final def maskMode(mode: Mode): Context = withMode(this.mode & mode)
     final def retractMode(mode: Mode): Context = withMode(this.mode &~ mode)
+
+    override def toString =
+      "Context(\n" +
+      (outersIterator map ( ctx => s"  owner = ${ctx.owner}, scope = ${ctx.scope}") mkString "\n")
   }
 
   /** A condensed context provides only a small memory footprint over
@@ -296,7 +300,7 @@ object Contexts {
     def withNewTyperState: this.type = withTyperState(typerState.fresh(isCommittable = true))
     def withExploreTyperState: this.type = withTyperState(typerState.fresh(isCommittable = false))
     def withPrinterFn(printer: Context => Printer): this.type = { this.printerFn = printer; this }
-    def withOwner(owner: Symbol): this.type = { this.owner = owner; this }
+    def withOwner(owner: Symbol): this.type = { assert(owner != NoSymbol); this.owner = owner; this }
     def withSettings(sstate: SettingsState): this.type = { this.sstate = sstate; this }
     def withCompilationUnit(compilationUnit: CompilationUnit): this.type = { this.compilationUnit = compilationUnit; this }
     def withTree(tree: Tree[_ >: Untyped]): this.type = { this.tree = tree; this }