Add debug error message for impossible case

Author: liufengyun

compiler/src/dotty/tools/dotc/transform/init/Objects.scala

@@ -298,8 +298,9 @@ class Objects {
               val rhs = target.defTree.asInstanceOf[ValOrDefDef].rhs
               eval(rhs, addr, target.owner.asClass, cacheResult = true)
 
-        case _: Fun =>
-          ???
+        case fun: Fun =>
+          report.error("unexpected tree in select a function, fun = " + fun.expr.show, source)
+          Result(Bottom, Nil)
 
         case RefSet(refs) =>
           val resList = refs.map(_.select(field, source))
@@ -429,8 +430,9 @@ class Objects {
           val res = addr.call(ctor, args, superType = NoType, source)
           Result(addr, res.errors)
 
-        case _: Fun =>
-          ??? // impossible
+        case fun: Fun =>
+          report.error("unexpected tree in instantiate a function, fun = " + fun.expr.show, source)
+          Result(Bottom, Nil)
 
         case RefSet(refs) =>
           val resList = refs.map(_.instantiate(klass, ctor, args, source))