cue: fail if non-optional definitions are bottom
This puts the implemenation on par with the spec.
The spec has been made slightly clearer to inidicate
that a non-optional failed definition is also a failure,
just like regular fields.
The code is copied from the previously used implementation
of walk. The old implementation just walked over
concrete values, so we needed another one to visit
all fields.
Change-Id: Ia20f83b7c1ba6299cb7372b018cf38d57038290b
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/3769
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/doc/ref/spec.md b/doc/ref/spec.md
index f07149d..4efc3a7 100644
--- a/doc/ref/spec.md
+++ b/doc/ref/spec.md
@@ -1010,7 +1010,7 @@
`a.f` and `b.f` are optional.
Any [references](#References) to `a` or `b`
in their respective field values need to be replaced with references to `c`.
-The result of a unification is bottom (`_|_`) if any of its required
+The result of a unification is bottom (`_|_`) if any of its non-optional
fields evaluates to bottom, recursively.
<!--NOTE: About bottom values for optional fields being okay.
