Better error message when an outer path is not found.

Author: odersky

src/dotty/tools/dotc/transform/ExplicitOuter.scala

@@ -314,7 +314,12 @@ object ExplicitOuter {
         val outerAccessorCtx = ctx.withPhaseNoLater(ctx.lambdaLiftPhase) // lambdalift mangles local class names, which means we cannot reliably find outer acessors anymore
         ctx.log(i"outer to $toCls of $tree: ${tree.tpe}, looking for ${outerAccName(treeCls.asClass)(outerAccessorCtx)} in $treeCls")
         if (treeCls == toCls) tree
-        else loop(tree.select(outerAccessor(treeCls.asClass)(outerAccessorCtx)).ensureApplied)
+        else {
+          val acc = outerAccessor(treeCls.asClass)(outerAccessorCtx)
+          assert(acc.exists,
+              i"failure to construct path from ${ctx.owner.ownersIterator.toList}%/% to `this` of ${toCls.showLocated};\n${treeCls.showLocated} does not have an outer accessor")
+          loop(tree.select(acc).ensureApplied)
+        }
       }
       ctx.log(i"computing outerpath to $toCls from ${ctx.outersIterator.map(_.owner).toList}")
       loop(This(ctx.owner.enclosingClass.asClass))