cue: tighten type

Change-Id: If6655c13825779eba87f7481e5a6d5985b259c6c
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2702
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/cue/types.go b/cue/types.go
index deb08d1..3f4fa3c 100644
--- a/cue/types.go
+++ b/cue/types.go
@@ -568,7 +568,7 @@
 	return Value{v.idx, &valueData{v.path, i, a}}
 }
 
-func (v Value) eval(ctx *context) value {
+func (v Value) eval(ctx *context) evaluated {
 	if v.path == nil || v.path.cache == nil {
 		panic("undefined value")
 	}